Slope is defined as dY / dX, where, in this case, your X-axis is time.
You could call it dY / dT.
d represents delta, or the difference between two Y values (or time-values).
Pick two points from your graph - T0,Y0, and T1,Y1
Your slope is (Y1 - Y0) / (T1 - T0)
If you're measuring volts, the slope is in Volts/second.
Simple, right?
The kicker is in picking the points to use. If you pick them close together, you run a larger risk of noise polluting your answer. If you pick them farther apart, you may miss some feature.
In your example data, if you pick T0 = 0.07, and T1 = 0.1, you'll get a negative slope.
If you pick T0 = 0.1 and T1 = 0.2 you'll get a positive slope.
If you pick T0 = 0.1 and T1 = 0.4, you'll get a lower positive slope.
All of them are correct, it depends on what you want to know.