Right-click on the graph and select "Lap Times". Then select the "Advanced" tab. Alternatively, if you have updated to PPP 4.0 (highly recommended) click the toolbar icon with all the squiggly lines and the tooltip of "Curves", and select that temperature be plotted.
Temperature is recorded alongside altitude (turn altitude off, and it won't record temperature either.) The HRM only records the temperature at each lap split (even though it's actively sensing the temperature constantly, to keep the altimeter calibrated.)