[Bug] 'encode' fails in heatmap series with calendar coordinate system. dimensions works instead

### What problem does this feature solve?

Hi,
it seems that currently the "calendar" coordinate system doesn't support any field to add a string to a date on a chart. This could be useful to add memos or notes to specific dates on the tooltip of charts like: scatter, effectScatter and graph,

### What does the proposed API look like?

Currenlty in Calendar.ts this dimensions are specified:

`

static getDimensionsInfo() {
return [{
name: 'time', type: 'time' as const
}, 'value'];
}
`
My proposal is to add a 'tooltip' or 'description' field to the data/dataset. This is a example of how should it be defined:
data: [['2025-01-01', 300, 'String']]