How can I approximate a signal
Assuming you want to remove the DC bias from your signal, a few thoughts come to mind. Your signal sequence looks like a noisy periodic wave riding on a DC bias. And the bias is trending upward in value from the beginning to the end of your sequence. Computing the average value of your signal sequence, a single value, and subtracting that value from your sequence will produce a sequence whose average value is very close to zero. But the upward bias trend will remain in your new sequence.

A process called "DC blocking" will remove both the average bias and the upward bias trend from your signal. If you have Matlab software, then use Matlab's detrend() command to implement DC blocking. If you don't have Matlab available then have a look at the following DC blocking networks (copied from Chapter 13 of my DSP book).



Other than different DC gains those above networks have the same frequency response. (Variable $\alpha$ is in the range of just greater than zero -to- one.) Note, those networks have very nonlinear phase response near zero Hz. If linear phase DC blocking is what you want then have a look at:
http://www.dsprelated.com/showarticle/58.php