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import { ChartPointer, MousePointer } from './types';
/**
* Computes the chart coordinates from the mouse event.
*
* The coordinates are relative to the top-left corner of the chart,
* where the top-left corner of the chart is (0, 0).
* Moving right, the x-coordinate increases, and moving down, the y-coordinate increases.
*
* The coordinates are rounded to the nearest integer and are including a CSS transform scale.
* So a chart that's scaled will return the same coordinates as a chart that's not scaled.
*
* @param event The mouse event from React event handlers
* @return chartPointer The chart coordinates relative to the top-left corner of the chart
*/
export const getChartPointer = (event: MousePointer): ChartPointer => {
const rect = event.currentTarget.getBoundingClientRect();
const scaleX = rect.width / event.currentTarget.offsetWidth;
const scaleY = rect.height / event.currentTarget.offsetHeight;
return {
/*
* Here it's important to use:
* - event.clientX and event.clientY to get the mouse position relative to the viewport, including scroll.
* - pageX and pageY are not used because they are relative to the whole document, and ignore scroll.
* - rect.left and rect.top are used to get the position of the chart relative to the viewport.
* - offsetX and offsetY are not used because they are relative to the offset parent
* which may or may not be the same as the clientX and clientY, depending on the position of the chart in the DOM
* and surrounding element styles. CSS position: relative, absolute, fixed, will change the offset parent.
* - scaleX and scaleY are necessary for when the chart element is scaled using CSS `transform: scale(N)`.
*/
chartX: Math.round((event.clientX - rect.left) / scaleX),
chartY: Math.round((event.clientY - rect.top) / scaleY),
};
};
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