Pre-1.16, Altair charts would not have tooltips unless they were explicitly defined. Now, tooltips are automatically-generated even if they are not defined. Setting theme=None does NOT revert this behavior. Setting tooltip= or tooltip=alt.Value(None) causes the chart to have a tooltip that simply reads “true”.
Steps to reproduce
Just make any chart and don’t specify tooltips. They’ll show up, even if you don’t want them. Even if you explicitly define them as blank as described above, you’ll still get a little box popping up saying “true”.
Several options, not mutually exclusive:
The tooltip behavior could revert when setting theme=None
The tooltip behavior could be reverted by a separate keyword option: e.g., autoTooltips=False
Setting tooltip= or tooltip=alt.Value(None) could appropriately leave the chart with NO tooltips, rather than the weird phantom “true”.