I included such an example in this post:
It doesn’t always show both, sometimes the graph just suddenly switches from the old sensor to the new one at the time the new one is started. I find it very useful to see both the old and new lines, because it helps me know whether the new sensor can be trusted yet. But I can imagine that Dexcom would find graphs like the one I posted somewhat embarrassing because of the drastic errors in the readings of the new sensor, and might want to disable that “so as not to confuse the patient” [or the FDA].
Oh, and my guess as to what is going on with the new sensor reading drastically low is that the Dexcom algorithm may be able to detect that it’s getting inconsistent or jittery measurements from the sensor, and for safety may choose to report the lowest credible BG rather than the most likely value. (Telling me that I’m 50 when I’m actually 90 won’t hurt me. Telling me that I’m 90 when I’m actually 50 can cause a car crash. Telling me I’m 140 when I’m actually 90 can lead me to overdose insulin and then go low.)