Can I visually obtain an equation from this curve?
http://en.wikipedia.org/wiki/RC_time_constant  

It looks like each block is 25ms and 1V for chnl2. Use an approximate rise time to calculate Tau:
rise time (20% to 80%): $$t_r \approx 1.4 \tau \approx \frac{0.22}{f_c}$$

Then use the difference in voltage from the start of the curve to the end to give you V0 for the charging equation:
  $$V(t) = V_0(1-e^{-t/ \tau}) $$

Use the discharge equation from that link to find the falling side equation. The tau for it will be different than for the rising one.