There's two fixes for this problem:
1) Use a modeless form. This includes adding document locks almost everywhere and is generally a pain in the butt.
2) Add a User Interaction, like this:
Private Sub PlotPreview()
Dim edUserInteraction As EditorUserInteraction = Application.DocumentManager.MdiActiveDocument.Editor.StartUserInteraction(Me)
' Do your plot preview
Catch ex As System.Exception
' Error handling
' Show the form
If Me.Visible = False Then Me.Show()
The EditorUserInteraction object allows your modal form to give up focus and allow the plot preview to do what it's supposed to.
Simple fix for a very frustrating problem.