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In this case does it matter though? The purpose of the chart is relative performance, which it conveys minus a legend. Though I would agree that if they aren't including the legend, they should remove the axis and standard range labels entirely.

It matters at a quick glance - is performance measured in images per millisecond or milliseconds per image? The answer to that question is obviously important to how you interpret the relative performance. Both metrics (units per time and time per unit) are used widely in computing.

I was initially confused by the chart and yes the text (and following table) make the units clear, but the chart should be standalone.

The legend is below each chart, with a table of the numeric results, with standard deviation. The code to reproduce the test is also provided, as well as the sample images.

They are milliseconds taken for the given task, as stated in the article.