As stated, or at least insinuated, that's the problem with bitmap file formats. If you try and enlarge them, it just makes each individual pixel bigger, so things get blurry. Vector graphics files don't have this issue, but aren't very widely used.
So in your case your best option would be to go back to the original chart and get a higher resolution photo of it. If that's not an option, you can either live with it being blurry, recreate the chart on your own, or try using a sharpening tool on a photo editing program like The GIMP or even something a bit simpler like Irfanview.
I apologize in advance if this isn't the right forum, but I just wanted to make sure I didn't post this on an inactive forum. Please move this if it's in the wrong place. I have a photo of a chart that is too small to read, but when I enlarge it it's too blurry. I tried printing it to possibly scan it bigger, but the copy was unreadable. Suggestions please?? Thanks. Ed.