Please could you provide a little more detail on what exactly you are trying to "copy". From the post it seems that what you want to copy is the plot's data from the small waveform graph to a larger waveform graph. If this is infact what you are trying to do, I would suggest configuring the appearance (such as the line color, line style, etc.) of the plot on the large waveform graph up front at design-time. To do this launch the collection editor for the Plots property of the large graph from the property grid in Visual Studio.
In your post, you also mentioned that you were trying to "copy" the plot "when you click on the small graph". But in the event handler code, you were attaching an event handler to the MouseHover event of the graph. The MouseHover event is raised when the mouse hovers over the control, whereas the MouseDown event is raised "when you click on the small graph". I am not sure which of these you were actually trying to handle, but for the solution below I have used the MouseDown event. Also notice that I am handling the PlotAreaMouseDown event rather than the MouseDown event.
AddHandler wfmFFTphase.PlotAreaMouseDown, AddressOf subPlotInBigGraph
Private Sub subPlotInBigGraph(ByVal sender As Object, ByVal e As MouseEventArgs)
'GetPlotAt returns the plot at which the mouse
'was clicked or null if the mouse was not clicked
'on a plot.
Dim hisPlot As WaveformPlot = CType(wfmFFTphase.GetPlotAt(e.X, e.Y), WaveformPlot)
If Not (hisPlot Is Nothing) Then
Me.Cursor = Cursors.WaitCursor
Me.SuspendLayout()
PlotFiltered.ClearData()
'Copy the small graph's data
Dim hisYData As Double() = hisPlot.GetYData()
'Plot the data on the bigger graph
PlotFiltered.Plots(0).PlotY(hisYData)
PlotFiltered.Visible = True
Me.ResumeLayout()
Me.Cursor = Cursors.Default
End If
End Sub