[Lazarus] TAChart DataPointCrossHairTool

[Werner Pamler]

To convert image coordinates (pixels within the chart rectangle which 
you, indeed, can obtain from the global mouse position converted to the 
local chart pixels) to "graph" coordinates you call the chart methods

* "XImageToGraph(x:Integer): Double" for x coordinates,
* "YImageToGraph(y:Integer): Double" for y coordinates, or
* "ImageToGraph(P:TPoint): TDoublePoint" (for both).

(or XGraphToImage() etc for the other direction).

If you do not use axis transformations, then graph coordinates directly 
refer to the units of your data. If, on the other hand, axis 
transformations are involved (logarithmic axes, multiple axes etc) the 
values on the axes may be different from the graph units used in an 
internal coordinate system. In this case, you have to link the series to 
the axis (by means of the AxisIndexX and AxisIndexY properties) and call 
the GraphToAxis or AxisToGraph methods of the AxisTransformation which 
is obtained from the method GetTransform of the axis. In your example of 
y values the transformation chain would be

   mouse point yi --> graph coords by yg := Chart.YImageToGraph(yi) -->  
axis coords by ya := Chart.LeftAxis.GetTransform.GraphToAxis(yg).

Here, ya refers to your input data, yi to the pixel data.

But since you don't use transformations YImageToGraph is sufficient.