I just had the most remarkable experience when I read your question ... a formula popped into my head that I haven't used or even thought about in years ! It's the formula for the charge on a capacitor in terms of the voltage across it:
Q = C · V
Charge = (capacitance) · (voltage)
This is exactly what we need in order to answer this question !
For a graph of charge vs voltage (which we have here !), that formula Q=CV is in slope-intercept form, and it says that the capacitance is the slope of the graph. How awesomely convenient !
The slope of the whole blue line on this graph is
Slope = (0.025 Coulomb) / (50 volts)
Slope = 0.0005
Capacitance = 0.0005 Farad or Capacitance = 500 microfarads
