For future readers who found this question while trying to save high resolution images from matplotlib as I am, I have tried some of the answers above and elsewhere, and summed them up here.
Best result: plt.savefig('filename.pdf')
and then converting this pdf to a png on the command line so you can use it in powerpoint:
pdftoppm -png -r 300 filename.pdf filename
OR simply opening the pdf and cropping to the image you need in adobe, saving as a png and importing the picture to powerpoint
Less successful test #1: plt.savefig('filename.png', dpi=300)
This does save the image at a bit higher than the normal resolution, but it isn't high enough for publication or some presentations. Using a dpi value of up to 2000 still produced blurry images when viewed close up.
Less successful test #2: plt.savefig('filename.pdf')
This cannot be opened in Microsoft Office Professional Plus 2016 (so no powerpoint), same with Google Slides.
Less successful test #3: plt.savefig('filename.svg')
This also cannot be opened in powerpoint or Google Slides, with the same issue as above.
Less successful test #4: plt.savefig('filename.pdf')
and then converting to png on the command line:
convert -density 300 filename.pdf filename.png
but this is still too blurry when viewed close up.
Less successful test #5: plt.savefig('filename.pdf')
and opening in GIMP, and exporting as a high quality png (increased the file size from ~100 KB to ~75 MB)
Less successful test #6: plt.savefig('filename.pdf')
and then converting to jpeg on the command line:
pdfimages -j filename.pdf filename
This did not produce any errors but did not produce an output on Ubuntu even after changing around several parameters.