A Scatter chart getting the X/Y value

The Line chart has no concept of an X axis scale. The values that you supply are simply spaced equally across the chart. The Scatter chart on the other hand does have an X axis scale, so you can have an uneven distribution of elements across the chart if you so wished.

Because the X axis is scaled this means that you can get the relevant X value from a click or mousemove event. You can also get the relevant Y value so you then have an (X,Y) coordinate pair. The code to achieve this is as follows. It uses the RGraph replacement for the mousemove event but this is not a requirement:

<script src="RGraph.common.core.js"></script>
<script src="RGraph.common.dynamic.js"></script>
<script src="RGraph.scatter.js"></script>

<div style="float: right; width: 600px">
    <canvas id="cvs" width="600" height="250">[No canvas support]</canvas>
    <div style="position: relative; left: 25px; width: 550px">
        X: <input type="text" id="x" /><br />
        Y: <input type="text" id="y" />
    </div>
</div>