Greek letters and math symbols don't work on vertical text such as Y axis titles.

A few peopl have encountered this problem using Prism Windows: You can enter the Greek letter when the vertical text is presented horizontally, but it goes away when the text flips back to vertical. The problem is apparent in Prism, as well as when the graph is exported or pasted into another application.

If this is a temporary problem, please restart your computer. This frees up "resources" on your computer, allowing Windows to cope with more fonts and more orientation of fonts.

If the problem occurs even with a freshly-booted computer, the problem probably is that your Symbols font is missing or corrupted or out-of-date. Find the file symbol.ttf in your \Windows\Fonts folder, and replace with a fresh copy ("borrowed" from another computer).