The left axis shows the Mask Mandate cases per 100K population (begins at 26 cases/100K population and decreases to 16 cases/100K population for Mask counties).
The right axis shows the No Mask Mandate cases per 100K population (goes sideways at 9 cases/100K) 3. The point of the slide is to show the different trends. Mask counties are trending down. No Mask counties are remaining the same (no improvement).
Each axis is labeled, and the same relative difference of 10 cases/100K is identical for each axis. For Mask counties, it goes from 15 to 25 cases/100K, and for No Mask counties, it goes from 4 to 14 cases/100K. This allows for a fair comparison for the "rate", or change of the trendline when comparing the two groups.
The take-home points here 1) mask mandates have dramatically decreased the rate of cases in the counties that have the mandates and 2) the general decline in the rate of cases for Kansas is being driven by the declines in these 16 mask counties, which by the way make up about 2/3 of the Kansas population which is why they have such an effect on the overall Kansas rate of cases.