Doing so ended up taking about an hour, during which I learned:
- that Excel can automatically calculate the difference between two dates
- that the Mac version of Excel is really, truly awful, but not much worse than the PC version of Excel. Its guesses about what I wanted to be in the X axis and what I wanted to be in the Y axis were almost terrible, and I only later learned that I needed a "Scatter" plot, a type of chart that, to judge by its near-inaccessibility, Microsoft is ashamed of; and
- that although OmniGraph Sketcher does only one thing, it does that extremely well. Creating the image below took approximately 2 minutes, which is less time than I spent fruitlessly googling for help on Excel commands.