Default gridline behavior for overlaid axes is ugly and hard to read

[emilykl]

When creating a chart with multiple y-axes (or x-axes) overlaid on the same plot, the default gridline behaviour is to show independent gridlines for both axes, which both doesn't look very nice, and also makes the plot somewhat difficult to read as it's not immediately obvious which gridlines correspond to which axis:

Codepen

Proposed solution

We already have an axis property, tickmode: 'sync', which addresses this issue. When tickmode is set to sync for the overlaid axis, that axis will use the same gridlines as the primary axis, and calculate its tick labels to match:

I propose that tickmode should default to 'sync' for overlaid axes (rather than the current default of 'auto').