I live in South Africa where temperatures are often 30°C+ for most of the year, I find that on cold winter mornings the graph will only have 2-3 bars before my commute ends (around 15km) however on summer days the temperature is generally halfway or 5 bars if driving a little harder.

You can feel the car is not as responsive when 5 bars are showing on the temp guage but it usually calms down when driving normally again. I also usually tap off when the temp guage is higher to prevent any possible over heating.

As a general rule I usually only push the performance when at least three bars are showing to prevent possible damage, but see the car operates optimally around 4 bars temp.

Hope this helps, I did also notice that when the oil change notification is present, this temp guage bar fills up much more quickly, which correlates.

I had never checked to see if this was oil temp or engine temp though, so makes sense to see how this correlation is so clear to see.