Marc,
The graph is being created. The <PIL.PngImagePlugin.PngImageFile image mode=RGBA size=640x480 at 0x3574BB0> is simply the text representation of the image. The actual image is sitting in the Excel cell where you created the code.
Unfortunately, it’s a very light graph with a white background, so it’s probably very hard to see that it’s actually there. If you click on the cell, an icon will appear above it which allows you to create a larger floating reference to the image.