The reason your not seeing anything is because the default plot style is only a line. But the line gets interupted at NaN’s so only multiple consequtive values will be plotted. And the latter doesnt happen in your case. You need to change the style of plotting, which depends on what you want to see.
For starters, try adding:
.plot(marker="o")
That should make all data points appear as circles. It easily gets cluttered so adjusting markersize, edgecolor etc might be usefull. Im not fully adjusted to how Pandas is using matplotlib so i often switch to matplotlib myself if plots get more complicated, eg:
plt.plot(df.R2.index.to_pydatetime(), df.R2, 'o-')