Describe the bug
After some recent changes #1212 the heatmap brush on time x scale is off by 1 cell

To Reproduce
Steps to reproduce the behavior:

1. Go to https://elastic.github.io/elastic-charts/?path=/story/heatmap-alpha--basic
2. Try to drag and select a singe cell
3. The selection will cover nothing or two cells minimum on the X axis

Expected behaviour
The brush should always be consistent with the dragged area

Screenshots

Version (please complete the following information):

• Elastic Charts: 31.0.0