My TA noticed that plots in Excel files do not always show up properly on the Teacher app. There were several times when the plot looks blank, or maybe has 1 or 2 point on it, but if we download their file from the browser and open it in Excel on the computer, we see the real plot with all the points and a trend line, etc.
I confirmed this is the case, and I also compared it to the view on the browser in Speed Grader, and it looks the same to me. That is, if the plot shows up blank in the Teacher app, it also does so on the browser on a PC. Thus, this is not specific to the app, but a general deficiency in the way that Canvas displays Excel files in grading mode. It is too bad, as this severely degrades the usefulness of the app for grading.
After posting I explored some more and I discovered that even in cases where I could see the plots, they did not match the plots in the downloaded file (at all). Its as if Canvas's Excel file interpreter just chose to plot the data in the way it wanted to, not how the student actually did it. In a way, this is far worse. At least, if there is no plot, I know something is wrong. If I see a plot, but it looks wrong, I am likely to count the student off.
Bottom Line: We cannot trust what we see in Canvas SpeedGrader (mobile or browser) when it displays Excel files, at least in terms of plots. (However, I cannot guarantee that it is limited to just plots. This may be a sign of something more fundamentally wrong.)