Dynamic Text Just Above An Altair Chart

In the chart produced by the code below, the user can hover over the line segment to display a point on the chart, and information about the point is shown above the chart. import

Solution 1:

Altair scales have both a domain and a range. The domain refers to the extent of the data, and the range refers to the extent of the visual scale.

In the case of x and y encodings, the range is measured in pixels from the left of the chart (for x) and from the top of the chart (for y). Setting an encoding valuealways refers to the range, not the domain.

So if you want the text to appear in a specific place regardless of the dataset, don't tie the text range position to values in the dataset. For example, you could do:

y=alt.value(-1)

and remove the dy setting from your mark properties. This would specify that you want the text baseline to be 1 pixel above the top of the y axis.