Charge discharge curve of a capacitor, Arduino
What's the purpose of the diode? If that's an LED, then it's clamping the voltage at around 1.9V, hiding most of the exponential curve that you are looking for.

In other words, the rising edge of your waveform is just the first part of an exponential curve that is asymptotically headed toward 5V, but gets stopped at 1.9V by the clamping action of the diode. The falling edge is the entire view of an exponential curve that starts at the diode clamp voltage, and is headed toward 0V &mdash; but gets interrupted by the next charge cycle.