In the Analytical Graphics Inc. video Low-Thrust Space Trajectory Design and Optimization the speaker Pradipto Ghosh of AGI shows a slide at around
07:00 shown below.
On it there is a plot with the vertical axis labeled $\Delta u / v_c$ and the horizontal axis labeled $v/v_c$.
What do the variables used in these axis labels represent?
Any other explanation of how the plot should be interpreted or used is appreciated!