mouseMove and CSS transforms don't get along

I have uPlots embedded in pages where I'm using a CSS transform to scale the entire page up or down. This mostly works, but the mouse position is not tracked correctly. Someone else hit the same problem and created a codepen: https://codepen.io/chrisdalke/pen/WNomELr look at the third chart and how the crosshair position is out of sync with the mouse pointer. Basically, uPlot uses page pixels but the whole uPlot is scaled by 2x so the cross hairs diverge from the pointer as you move right and down from the top-left corner. MouseMove should use "element coordinates" that are scaled by the browser just like the element is scaled.

(I have no association with the user that created that codepen, I just bumped into it as I was going to create my own codepen, saved me a bunch of work :-).

I played around a bit and the following fixes the issue for me. In cacheMouse https://github.com/leeoniya/uPlot/blob/master/src/uPlot.js#L2737-L2740 replace by:

		if (e != null) {
			_l = e.offsetX;
			_t = e.offsetY;
		}

I'm not a browser event whizz so I don't know what else this might break. The mozilla docs https://developer.mozilla.org/en-US/docs/Web/API/MouseEvent/offsetX are "clear as mud" to me...

Thanks for an amazing lib overall!!!

Start in src/uPlot.js around cacheMouse at lines 2737-2740, then reproduce the third chart in the linked CodePen with a 2x CSS transform. Compare the event coordinate choices and verify that the crosshair stays aligned with the pointer as it moves across the chart, without breaking normal mouse tracking.