Using a CrosshairLayer with multiple series is a cool feature. It is displaying the related Y-values to the same X-data point. This is verry useful for application users to compare values of e.g. time series. Now this feature currently is only usable for charts with CategoryAxes (see below).

If you want to implement CrosshairLayer with NumericXAxis and ScatterLineSeries you will result in a somehow “strange” unexpected behavior. The crossing positions are not calculated by related X, but by some nearest point of line to mouse cursor algorithm (see below and attached sample project).

CrosshairTest.zip

Please give us the opportunity to have a X synchronized ChrosshairLayer. Perhaps you could add a ItemSearchMode="ClosestX" property to the scatter series or to the CrosshairLayer.