如何在altair分层图中添加图例

Question

所以我有以下代码：

workload_df = pd.DataFrame({
'index': pd.to_datetime(['01.02.2010', '01.03.2010', '01.04.2010']),
'measure': [100, 90, 120],
'measure_max': [80, 100, 150],
})

measure_max_plot = alt.Chart(workload_df).mark_bar(color = 'lightgreen', text = 'measure_max').encode(
    alt.X('index', title = '', axis = alt.Axis(labelAngle = -45, labelOverlap = False)),
    alt.Y('measure_max', title = '')
)

measure_plot = alt.Chart(workload_df).mark_bar(text = 'measure').encode(
  x = alt.X('index', title = 'X', axis = alt.Axis(labelAngle = -45, labelOverlap = False)), 
  y = alt.Y('measure', title = 'Y'),

  color=alt.condition(
    alt.datum.measure > alt.datum.measure_max,  
    alt.value('red'),     
    alt.value('steelblue')   
  )
)
altair_plot = alt.layer(measure_max_plot, measure_plot)
st.altair_chart(altair_plot, use_container_width=True)

我已经尝试使用以下解决方案添加图例：Add legend to line & bars to Altair chart without using size/color

但是总是得到一个奇怪的错误，或者得到一个没有任何绘图数据的图例。

有人能帮我吗？

Answer 1

为了将图例添加到图表中，您需要图例将表示的编码。例如，下面是如何添加将生成颜色图例的颜色编码的方法：

measure_max_plot = alt.Chart(workload_df).transform_calculate(
    color='"measure_max"'
).mark_bar(text = 'measure_max').encode(
    alt.X('index', title = '', axis = alt.Axis(labelAngle = -45, labelOverlap = False)),
    alt.Y('measure_max', title = ''),
    alt.Color('color:N')
)

measure_plot = alt.Chart(workload_df).transform_calculate(
  color="datum.measure > datum.measure_max ? 'bigger' : 'smaller'"
).mark_bar(text = 'measure').encode(
  x = alt.X('index', title = 'X', axis = alt.Axis(labelAngle = -45, labelOverlap = False)), 
  y = alt.Y('measure', title = 'Y'),
  color = alt.Color('color:N', scale=alt.Scale(range=['red', 'lightgreen', 'steelblue']))
)
altair_plot = alt.layer(measure_max_plot, measure_plot)

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请注意，默认情况下，色标在两个分层图表之间共享。可以使用Scale & guide resolution应用编程接口对此行为进行微调。