Sales Graph Not Correctly Labelled

We've noticed for the last year or so that the sales graph in the admin console is not labelling current vs last year sales correctly. For the two lines, both are labelled identically for the current year (see attached screenshots).

Any ideas out there as to why?

[attachment=12789:Screen Shot 2017-11-22 at 10.48.17 AM.png]

[attachment=12790:Screen Shot 2017-11-22 at 10.48.23 AM.png]

Screen Shot 2017-11-22 at 10.48.17 AM.png

Screen Shot 2017-11-22 at 10.48.23 AM.png