bfix: Fix histograms (#58)

* Fix Issue #57. We were failing to clear some state in the histogram, leading to errors the second/third/etc time it was rendered.

* Add reference images for the new test

Author: karwa

Sources/SwiftPlot/Histogram.swift

@@ -269,6 +269,7 @@ extension Histogram: HasGraphLayout {
             histogramSeries.scaledBinFrequency.append(frequency*scaleYInv + origin.y)
         }
         for index in 0..<histogramStackSeries.count {
+            histogramStackSeries[index].scaledBinFrequency.removeAll()
             for j in 0..<histogramStackSeries[index].binFrequency.count {
                 let frequency = Float(histogramStackSeries[index].binFrequency[j])
                 histogramStackSeries[index].scaledBinFrequency.append(frequency*scaleYInv + origin.y)

Tests/SwiftPlotTests/Histogram/histogram-regression-tests.swift

@@ -0,0 +1,37 @@
+import SwiftPlot
+import SVGRenderer
+#if canImport(AGGRenderer)
+import AGGRenderer
+#endif
+#if canImport(QuartzRenderer)
+import QuartzRenderer
+#endif
+
+extension HistogramTests {
+  
+  /// Regression test [#57]. We used to fail to clear the histogram state between renders,
+  /// leading to slight offsets in the subsequent AGG and CG renders.
+  func testHistogramStackedStepLineJoins() throws {
+    let fileName = "_reg_57_histogram_stacked_step_line_joins"
+    
+    let histogram = Histogram<Float>(isNormalized: false, enableGrid: true)
+    histogram.addSeries(data: [5], bins: 10, label: "Plot 1", color: .blue, histogramType: .step)
+    histogram.addStackSeries(data: [6], label: "Plot 2", color: .orange)
+    histogram.plotTitle = PlotTitle("HISTOGRAM STACKED STEP")
+    histogram.plotLabel = PlotLabel(xLabel: "X", yLabel: "Frequency")
+      
+    let svg_renderer = SVGRenderer()
+    try histogram.drawGraphAndOutput(fileName: self.svgOutputDirectory+fileName,
+                                     renderer: svg_renderer)
+    #if canImport(AGGRenderer)
+    let agg_renderer = AGGRenderer()
+    try histogram.drawGraphAndOutput(fileName: self.aggOutputDirectory+fileName,
+                                     renderer: agg_renderer)
+    #endif
+    #if canImport(QuartzRenderer)
+    let quartz_renderer = QuartzRenderer()
+    try histogram.drawGraphAndOutput(fileName: self.coreGraphicsOutputDirectory+fileName,
+                                     renderer: quartz_renderer)
+    #endif
+  }
+}

Tests/SwiftPlotTests/Reference/agg/_reg_57_histogram_stacked_step_line_joins.png

@@ -38,6 +38,7 @@ extension HistogramTests {
         ("testHistogram", testHistogram),
         ("testHistogramStacked", testHistogramStacked),
         ("testHistogramStackedStep", testHistogramStackedStep),
+        ("testHistogramStackedStepLineJoins", testHistogramStackedStepLineJoins),
         ("testHistogramStep", testHistogramStep),
     ]
 }