It's tough to give a detailed answer without seeing the structure of your data and preliminary code, but here are a few suggestions:
1) Convert your TIMESTAMPs to datetime.
2) You could use varfun to plot, using SITE_ID as a grouping variable. Something like
p = varfun(@(dt,temp) plot(dt,temp), t, 'InputVariables',{'TIMESTAMP','TEMP_C'},'GroupingVariables','SITE_ID')
It will actually return a table with the line objects for the charts. If you use hold on, you can get all the plots on one chart.
3) If you want subplots, you're right that you're probably better off iterating over the unique values of SITE_ID.
