! \internal Calculates the (minimum) distance (in pixels) the graph's representation has from the given \a pixelPoint in pixels. This is used to determine whether the graph was clicked or not, e.g. in \ref selectTest. If either the graph has no data or if the line style is \ref lsNone and the scatter style's shape is \ref QCPScatterStyle::ssNone (i.e. there is no visual representat