Rolf,

I believe the best solution to your question would be to take the data
found after the certain X value and put both the X data and the Y data
(after that value) into another worksheet or into new columns in your
current worksheet. If you choose the latter you will need to enable
the Multiple X Columns checkbox found in the Worksheet Display Control
(Format: Worksheet).

Once you have the data after that X value into another worksheet (or
new columns), you need to create a two layered graph. To do so, first
go to the worksheet (or part of the worksheet) that contains the
first set of X and Y data. Highlight the X and Y column with your
mouse and choose Plot: Line (or whatever it is that you want). After
you do this, go to Edit: New Layers: Linked Right Y. This places a
new layer onto the graph window with a Right Y axis that is linked
to the same X axis as the Left Y axis. Then, go to the upper left
corner of your graph and double-click on the grey number 2. Once
you have done this you will see that the Layer 2 dialog box comes up.
In the available datasets list highlight the dataset that corresponds
to the data found AFTER the X value. Click on the right arrow (=>)
key to move this dataset into the layer. Click OK. At this point
you may have to rescale the X axis to get both plots to appear in the
graph.