2.2. Scatter plots

So far, we have looked only at line plots. In this section, we branch out a bit and look at scatter plots. The Radian directives for all plot types work more or less the same way, so we’ll be able to get going very quickly based on the knowledge acquired using the <lines> directive for line plots.

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The <points> directive

In Radian, scatter plots are generated using the <points> directive. The coordinates of points to be plotted are specified using x and y attributes, as for the <lines> directive. The primary difference between using <points> and <lines> is in the range of paint attributes that can be given for the markers displayed at each of the plot points. Markers have a shape and size, and can be filled and stroked (so have colours and opacities for both as well as a stroke width).

<plot height=300 aspect=1>
  <points x="[[iris.sepal_length]]" y="[[iris.petal_length]]"
          marker-size=25 stroke="none" fill="red"></points>
</plot>

<plot-data name="iris" format="csv" src="/data/iris.csv"
           cols="sepal_length,sepal_width,petal_length,petal_width,species">
  <metadata name="sepal_length" label="Sepal length" units="mm"></metadata>
  <metadata name="sepal_width" label="Sepal width" units="mm"></metadata>
  <metadata name="petal_length" label="Petal length" units="mm"></metadata>
  <metadata name="petal_width" label="Petal width" units="mm"></metadata>
</plot-data>

Marker type is given using the marker attribute. Marker size is set by the marker-size attribute, which gives a measure of the area in square pixels of the marker. Colours are set with the stroke and fill attributes.

As a more advanced example of what can be done, the following example assigns marker type and fill colour based on the discrete values of the species field in our data. This uses Radian palettes, a topic that we won’t discuss much, but you can see more or less what to do (all the details are in the reference manual).

<plot height=300 aspect=1>
  <points x="[[iris.sepal_length]]" y="[[iris.petal_length]]"
          marker="[[spmark(iris.species)]]"
          marker-size=25 fill="[[spcol(iris.species)]]"></points>
</plot>

<palette name="spmark" type="discrete">
  "I. setosa"     circle
  "I. versicolor" square
  "I. virginica"  diamond
</palette>

<palette name="spcol" type="discrete">
  "I. setosa"     red
  "I. versicolor" green
  "I. virginica"  blue
</palette>

<plot-data name="iris" format="csv" src="/data/iris.csv"
           cols="sepal_length,sepal_width,petal_length,petal_width,species">
  <metadata name="sepal_length" label="Sepal length" units="mm"></metadata>
  <metadata name="sepal_width" label="Sepal width" units="mm"></metadata>
  <metadata name="petal_length" label="Petal length" units="mm"></metadata>
  <metadata name="petal_width" label="Petal width" units="mm"></metadata>
</plot-data>

"I. setosa" circle; "I. versicolor" square; "I. virginica" diamond

"I. setosa" red; "I. versicolor" green; "I. virginica" blue

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