[Feature] Be able to set the cell's backgroundColor when chart type is heatmap

[Monica-321]

What problem does this feature solve?

When I use the Heatmap chart, i need to set a backgroundColor when cell's value is null

What does the proposed API look like?

{ type: 'heatmap', data: chartData, itemStyle: { backgroundColor: '#A8A8A8', }, ... }

[Monica-321]

oh I find that if i use splitArea: { show: true, areaStyle: { color: ['#ccc'], }, }, in yAxis, it may behave like what i want!