Hi Frank,
The slope from the above graph is around 0.75 mV/C, which was collected using a 5V external reference.
There seem to be a difference in the temp sensor output as you have noticed and as previously mentioned when using an internal reference of 4.096V vs. 5V external reference. The typical temp drift of this internal reference is 10 ppm/C, which also affects the temp sensor output.
The temp sensor is a value-added feature and we can't calibrate it internally due to offset and gain trim issues unfortunately.
Regards,
Maithil