Sorry, finally looked at the rest of the question.
It sounds like maybe you have Report Generation Toolkit? If so, keep track of how much data you're sending to the chart and as you approach (the history length*Integer) take an image of the control and add it to your excel report via "excel insert object.vi".
If you go the graph route, you could wait until the end of data acquisition and take a single auto-zoomed image of the entire data set. Or, you could use a FOR loop to set the x-axis min and max (via property node) and take a snapshot after each step in order to generate a series of zoomed in images.
