Merge pull request #330 from AaltoSciComp/course-2025/vega-altair

content/vega-altair: Added some new content for the 2025 run

Author: simo-tuomisto

content/plotting-vega-altair.md

@@ -318,6 +318,62 @@ Monthly temperature ranges for two cities in Norway.
 ::::
 
 
+## Customizing channels for extra beauty
+
+All of the visual channels can be modified with multiple different parameters.
+The shorthand notation for specifying the channels can be modified to use channel
+objects initialized by the user.
+
+For the whole list of available channels and corresponding classes, see
+[this page](https://altair-viz.github.io/user_guide/encodings/channels.html).
+
+
+For example, we can rename channels to make them more beautiful:
+
+```python
+alt.Chart(data_monthly).mark_area(opacity=0.5).encode(
+    x=alt.X("yearmonth(date):T", title="Date"),
+    y=alt.Y("max temperature", title="Temperature range (min-max)"),
+    y2="min temperature",
+    color=alt.Color("name", title="Station name"),
+)
+```
+
+:::{figure} plotting-vega-altair/channels-renamed.svg
+:alt: Monthly temperature ranges for two cities in Norway.
+
+Monthly temperature ranges for two cities in Norway.
+:::
+
+## Saving charts
+
+Saving charts is easy: you just call the
+[chart.save()](https://altair-viz.github.io/user_guide/saving_charts.html)-method.
+
+```python
+alt.Chart(data_monthly).mark_area(opacity=0.5).encode(
+    x=alt.X("yearmonth(date):T", title="Date"),
+    y=alt.Y("max temperature", title="Temperature range (min-max)"),
+    y2="min temperature",
+    color=alt.Color("name", title="Station name"),
+).save("temperature_range.png", ppi=144)
+```
+
+Altair supports png, svg and pdf saving. For png files you can adjust the image
+resolution by giving an extra keyword argument (`ppi`).
+
+It also supports exporting the image as
+[HTML](https://altair-viz.github.io/user_guide/saving_charts.html#html-format)
+or as
+[JSON](https://altair-viz.github.io/user_guide/saving_charts.html#json-format)
+that can be embedded
+to any web page.
+
+You can even use
+[chart.to_url()](https://altair-viz.github.io/user_guide/saving_charts.html#sharable-url)-method
+to get a shareable url that you can view in an online editor (remember that
+the URL will also contain some of the data used to create the plot).
+
 ## More fun with visual channels
 
 Now we will try to **plot the daily data and look at snow depths**. We first
@@ -419,6 +475,38 @@ Snow depth (in cm) for the months December 2022 to May 2023 for two cities in No
 :::
 
 
+## Data transformations
+
+Vega-Altair also provides many different
+[data transformations](https://altair-viz.github.io/user_guide/transform/index.html)
+that you can use to quickly modify plots.
+
+For example, lets plot weekly average snowfall:
+
+```python
+alt.Chart(data_daily).mark_bar().encode(
+    x="week(date)",
+    y="mean(snow depth)",
+    color="max temperature",
+    column="name",
+)
+```
+
+Here we have used the `mean`-transform from
+[Aggregate](https://altair-viz.github.io/user_guide/transform/aggregate.html)-transformations
+and `week`-transform from
+[TimeUnit](https://altair-viz.github.io/user_guide/transform/timeunit.html)-transformations.
+
+Vega-Altair will then automatically do a group by-operation over weekly periods
+across aggregated columns.
+
+:::{figure} plotting-vega-altair/weekly-snow-depth.svg
+:alt: Weekly mean snow depth (in cm) for the months December 2022 to May 2023 for two cities in Norway. Colored by daily max temperature.
+
+Weekly mean snow depth (in cm) for the months December 2022 to May 2023 for two cities in Norway. Colored by weekly max temperature.
+
+:::
+
 ## Themes
 
 In [Vega-Altair](https://altair-viz.github.io/) you can change the theme and