feat: add DensityMap trace for the MapLibre map subplot (#416)

Add the modern MapLibre-based `densitymap` trace type, the density
counterpart to `ChoroplethMap`. It draws a weighted kernel-density
heatmap from lat/lon points (optional per-point weight z) onto the
`layout.map` subplot, with full color-scale (colorscale, colorbar,
showscale, reversescale, autocolorscale, coloraxis, zmin/zmax/zmid)
and hover (text, hovertext, hoverinfo, hovertemplate, hoverlabel,
customdata) support. Unlike ChoroplethMap it has no marker/selection.

Registers the trace: DensityMap variant on PlotType (serializes to
"densitymap"), module + type re-exports in traces/mod.rs and lib.rs.
Includes a serialize_density_map unit test and a doctest.

Signed-off-by: Andrei Gherghescu <8067229+andrei-ng@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jqnatividad

CHANGELOG.md

@@ -11,6 +11,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/) a
 - [[#410](https://github.com/plotly/plotly.rs/pull/410)] Add `Choropleth` (geo subplot) and `ChoroplethMap` (MapLibre `map` subplot) trace types, with a `LocationMode` enum and a dedicated `choropleth::Marker`; add the MapLibre `map` subplot via `LayoutMap`/`MapStyle`/`MapBounds`
 - [[#410](https://github.com/plotly/plotly.rs/pull/410)] Add `LayoutGeo` options: `fitbounds` (`GeoFitBounds`), `resolution` (`GeoResolution`, 1:110M/1:50M base-layer detail), and `bgcolor`
 - [[#412](https://github.com/plotly/plotly.rs/pull/412)] Add `Violin` trace type with box, mean line, KDE span, and split/grouped support
+- [[#414](https://github.com/plotly/plotly.rs/issues/414)] Add `DensityMap` (MapLibre `map` subplot) trace type — density heatmaps with full color-scale and hover support
 
 ### Changed
 

plotly/src/common/mod.rs

@@ -230,6 +230,7 @@ pub enum PlotType {
     Sankey,
     Surface,
     DensityMapbox,
+    DensityMap,
     Table,
     Pie,
     Treemap,

plotly/src/lib.rs

@@ -60,14 +60,14 @@ pub use layout::Layout;
 pub use plot::{Plot, Trace, Traces};
 // Also provide easy access to modules which contain additional trace-specific types
 pub use traces::{
-    box_plot, choropleth, choropleth_map, contour, heat_map, histogram, image, mesh3d, sankey,
-    scatter, scatter3d, scatter_mapbox, sunburst, surface, treemap, violin,
+    box_plot, choropleth, choropleth_map, contour, density_map, heat_map, histogram, image, mesh3d,
+    sankey, scatter, scatter3d, scatter_mapbox, sunburst, surface, treemap, violin,
 };
 // Bring the different trace types into the top-level scope
 pub use traces::{
-    Bar, BoxPlot, Candlestick, Choropleth, ChoroplethMap, Contour, DensityMapbox, HeatMap,
-    Histogram, Image, Mesh3D, Ohlc, Pie, Sankey, Scatter, Scatter3D, ScatterGeo, ScatterMapbox,
-    ScatterPolar, Sunburst, Surface, Table, Treemap, Violin,
+    Bar, BoxPlot, Candlestick, Choropleth, ChoroplethMap, Contour, DensityMap, DensityMapbox,
+    HeatMap, Histogram, Image, Mesh3D, Ohlc, Pie, Sankey, Scatter, Scatter3D, ScatterGeo,
+    ScatterMapbox, ScatterPolar, Sunburst, Surface, Table, Treemap, Violin,
 };
 
 pub trait Restyle: serde::Serialize {}

plotly/src/traces/density_map.rs

@@ -0,0 +1,207 @@
+//! Density heatmap trace for the MapLibre `map` subplot.
+
+use plotly_derive::FieldSetter;
+use serde::Serialize;
+
+use crate::common::{
+    ColorBar, ColorScale, Dim, HoverInfo, Label, LegendGroupTitle, PlotType, Visible,
+};
+use crate::private::{NumOrString, NumOrStringCollection};
+use crate::Trace;
+
+/// Construct a density heatmap trace drawn on the MapLibre `map` subplot
+/// (configured via [`LayoutMap`](crate::layout::LayoutMap)).
+///
+/// `DensityMap` is the modern MapLibre-based counterpart to
+/// [`DensityMapbox`](crate::DensityMapbox): it draws a weighted kernel-density
+/// heatmap from `lat`/`lon` points (with an optional per-point weight `z`) onto
+/// the `layout.map` subplot.
+///
+/// # Examples
+///
+/// ```
+/// use plotly::DensityMap;
+///
+/// let trace = DensityMap::new(vec![45.5017], vec![-73.5673], vec![1.0])
+///     .radius(20)
+///     .name("montreal");
+/// ```
+#[serde_with::skip_serializing_none]
+#[derive(Serialize, Clone, Debug, FieldSetter)]
+#[field_setter(box_self, kind = "trace")]
+pub struct DensityMap<Lat, Lon, Z>
+where
+    Lat: Serialize + Clone,
+    Lon: Serialize + Clone,
+    Z: Serialize + Clone,
+{
+    #[field_setter(default = "PlotType::DensityMap")]
+    r#type: PlotType,
+    /// Sets the trace name. The trace name appears as the legend item and on
+    /// hover.
+    name: Option<String>,
+    /// Determines whether or not this trace is visible.
+    visible: Option<Visible>,
+    /// Determines whether or not an item corresponding to this trace is shown
+    /// in the legend.
+    #[serde(rename = "showlegend")]
+    show_legend: Option<bool>,
+    /// Sets the legend rank for this trace.
+    #[serde(rename = "legendrank")]
+    legend_rank: Option<usize>,
+    /// Sets the legend group for this trace.
+    #[serde(rename = "legendgroup")]
+    legend_group: Option<String>,
+    /// Set and style the title to appear for the legend group.
+    #[serde(rename = "legendgrouptitle")]
+    legend_group_title: Option<LegendGroupTitle>,
+    /// Assigns id labels to each datum.
+    ids: Option<Vec<String>>,
+
+    /// Sets the latitude coordinates (in degrees North).
+    lat: Option<Vec<Lat>>,
+    /// Sets the longitude coordinates (in degrees East).
+    lon: Option<Vec<Lon>>,
+    /// Sets the points' weight. For example, a value of 10 would be equivalent
+    /// to having 10 points of weight 1 in the same spot.
+    z: Option<Vec<Z>>,
+    /// Sets the radius of influence of one `lat`/`lon` point in pixels.
+    /// Increasing the value makes the density map smoother, but less detailed.
+    radius: Option<u8>,
+    /// Sets the opacity of the trace.
+    opacity: Option<f64>,
+
+    /// Sets the text elements associated with each (lat,lon,z) triplet.
+    text: Option<Dim<String>>,
+    /// Sets the hover text elements associated with each (lat,lon,z) triplet.
+    #[serde(rename = "hovertext")]
+    hover_text: Option<Dim<String>>,
+    /// Determines which trace information appears on hover.
+    #[serde(rename = "hoverinfo")]
+    hover_info: Option<HoverInfo>,
+    /// Template string used for rendering the information that appears on the
+    /// hover box.
+    #[serde(rename = "hovertemplate")]
+    hover_template: Option<Dim<String>>,
+    #[serde(rename = "hovertemplatefallback")]
+    hover_template_fallback: Option<Dim<String>>,
+    /// Properties of the hover label.
+    #[serde(rename = "hoverlabel")]
+    hover_label: Option<Label>,
+
+    /// Determines whether or not the color domain is computed with respect to
+    /// the input data (here in `z`) or the bounds set in `zmin` and `zmax`.
+    #[serde(rename = "zauto")]
+    z_auto: Option<bool>,
+    /// Sets the lower bound of the color domain.
+    #[serde(rename = "zmin")]
+    z_min: Option<f64>,
+    /// Sets the upper bound of the color domain.
+    #[serde(rename = "zmax")]
+    z_max: Option<f64>,
+    /// Sets the mid-point of the color domain.
+    #[serde(rename = "zmid")]
+    z_mid: Option<f64>,
+    /// Sets the colorscale.
+    #[serde(rename = "colorscale")]
+    color_scale: Option<ColorScale>,
+    /// Determines whether the colorscale is a default palette.
+    #[serde(rename = "autocolorscale")]
+    auto_color_scale: Option<bool>,
+    /// Reverses the color mapping if true.
+    #[serde(rename = "reversescale")]
+    reverse_scale: Option<bool>,
+    /// Determines whether or not a colorbar is displayed for this trace.
+    #[serde(rename = "showscale")]
+    show_scale: Option<bool>,
+    /// Properties of the colorbar.
+    #[serde(rename = "colorbar")]
+    color_bar: Option<ColorBar>,
+    /// Sets a reference to a shared color axis.
+    #[serde(rename = "coloraxis")]
+    color_axis: Option<String>,
+
+    /// Sets a reference to the `map` subplot this trace is drawn on. Defaults
+    /// to `"map"`.
+    subplot: Option<String>,
+    /// Determines if this trace's layer is inserted below the layer with the
+    /// specified ID. By default, the layer is inserted above every existing
+    /// layer.
+    below: Option<String>,
+
+    /// Assigns extra meta information associated with this trace.
+    meta: Option<NumOrString>,
+    /// Assigns extra data to each datum.
+    #[serde(rename = "customdata")]
+    custom_data: Option<NumOrStringCollection>,
+    /// Controls persistence of some user-driven changes to the trace.
+    #[serde(rename = "uirevision")]
+    ui_revision: Option<NumOrString>,
+}
+
+impl<Lat, Lon, Z> DensityMap<Lat, Lon, Z>
+where
+    Lat: Serialize + Clone + std::default::Default,
+    Lon: Serialize + Clone + std::default::Default,
+    Z: Serialize + Clone + std::default::Default,
+{
+    pub fn new(lat: Vec<Lat>, lon: Vec<Lon>, z: Vec<Z>) -> Box<Self> {
+        Box::new(Self {
+            lat: Some(lat),
+            lon: Some(lon),
+            z: Some(z),
+            ..Default::default()
+        })
+    }
+}
+
+impl<Lat, Lon, Z> Trace for DensityMap<Lat, Lon, Z>
+where
+    Lat: Serialize + Clone,
+    Lon: Serialize + Clone,
+    Z: Serialize + Clone,
+{
+    fn to_json(&self) -> String {
+        serde_json::to_string(&self).unwrap()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use serde_json::{json, to_value};
+
+    use super::*;
+    use crate::common::ColorScalePalette;
+
+    #[test]
+    fn serialize_density_map() {
+        let trace = DensityMap::new(vec![45.5017], vec![-73.5673], vec![1.0])
+            .name("montreal")
+            .radius(20)
+            .opacity(0.5)
+            .hover_text_array(vec!["Montreal"])
+            .hover_info(HoverInfo::Text)
+            .color_scale(ColorScale::Palette(ColorScalePalette::Viridis))
+            .show_scale(true)
+            .subplot("map")
+            .below("");
+
+        let expected = json!({
+            "type": "densitymap",
+            "lat": [45.5017],
+            "lon": [-73.5673],
+            "z": [1.0],
+            "name": "montreal",
+            "radius": 20,
+            "opacity": 0.5,
+            "hovertext": ["Montreal"],
+            "hoverinfo": "text",
+            "colorscale": "Viridis",
+            "showscale": true,
+            "subplot": "map",
+            "below": "",
+        });
+
+        assert_eq!(to_value(trace).unwrap(), expected);
+    }
+}

plotly/src/traces/mod.rs

@@ -6,6 +6,7 @@ mod candlestick;
 pub mod choropleth;
 pub mod choropleth_map;
 pub mod contour;
+pub mod density_map;
 mod density_mapbox;
 pub mod heat_map;
 pub mod histogram;
@@ -31,6 +32,7 @@ pub use candlestick::Candlestick;
 pub use choropleth::Choropleth;
 pub use choropleth_map::ChoroplethMap;
 pub use contour::Contour;
+pub use density_map::DensityMap;
 pub use density_mapbox::DensityMapbox;
 pub use heat_map::HeatMap;
 pub use histogram::Histogram;