Measuring the capacity of a capacitor with arduino?
Use one of the Arduino's analog inputs to take measurements.  The impedance of the ADC input should be large enough so it will not affect your measurements.

Depending on the X-axis (time scale) of your spreadsheet, you will want to take readings at a fixed rate, say every 10 ms or whatever you choose.  Then save the values to RAM.

Connect the Arduino to a PC using a USB to UART bridge like this one from SparkFun.  The UART will show up as a virtual COMM port on the PC.

Download a terminal program like RealTerm, and open it to the port indicated in the Device Manager of your PC corresponding to the virtual COMM port of USB cable.

In the Arduino, convert each reading to an ASCII number followed by a carriage return/line feed (cr/lf), and output to the UART.  When done copy the readings from the terminal program and input them to Excel.