When you do a "Print Preview" or "Print" of the statistics or the report for a quiz that contains a vertical bar graph, neither the grid lines nor the percent labels (0%, 25%, 50%, 75%, 100%) show in the bar graph.
Sample Bar Graph Viewed in Quiz Statistics or Report
Printout of Sample Bar Graph
This is a known issue.
To obtain a print out of the statistics or report containing a bar graph, you can take a screen shot, paste it in Microsoft Paint, and print it from any application capable of viewing and printing images. If you have a screen capturing application such as "SnagIt", you can use that application to create an image of the statistics or report and then print the image.