guessing capacitor behaviour correctly
As current flows through a capacitor, it charges up and develops a voltage across it. This voltage takes up some of the voltage drop from the source that is available to the rest of the circuit.
That's why you have that negative slope. As the cap develops a voltage, less voltage is available to power the other components in your circuit.
If the cap ever manages to charge up to match the source voltage, then zero volts is available to the rest of the circuit and no current can flow but this circuit is changing too often, too quickly for the capacitor ever reach that point.