Trouble with understanding a plot (left panel, X-axis)

The plot is from Searchfor star cluster age gradients across spiral arms of three LEGUS disc galaxies | Monthly Notices of the Royal Astronomical Society | Oxford Academic.
I am having trouble understanding the X-axis in the left panel; the figure is described in the paper as that it “…shows the normalised distribution of the azimuthal distance of star clusters in the three age bins from their closest spiral arm ridge line in NGC 1566, M51a, and NGC 628…”

But I am confused regarding the azimuthal distance. How is it defined with respect to the spiral?

Is it like this?