It seems the Heatmap chart type in Vaadin Charts 3.0.0 mangles the colors–whether using discrete Stops or minColor + maxColor on the ColorAxis–when the chart’s Legend is disabled. I’ve tested this by adding a button to the page that simply toggles Legend.enabled and redraws the chart.
Legend enabled: Colors display as intended
Legend disabled: Colors become “muted” (red turns to brownish-orange; orange turns to brown; green stays about the same)
In the end my goal is to produce a heatmap with no visible legend, or at least a legend with no numeric labels, but it seems overriding the Legend label formatter has no effect.
As an aside, one way to hide the legend data labels is to set a format string of " "; “” reverts to the default, and I didn’t have any luck with further efforts via a custom JS formatter.