When changing the time scale ("zoom") on a graph, the value for "Last" changes depending on the zoom. "Last" should not change depending on zoom because it should always be the last value.

Try to open a simple graphs (item should have history for 4+ days, update interval should be 30 seconds for example) on two tabs: one with one hour period and second with for example 3 days period.
Graph on bot tabs should get data from History.
Values for the item should be dynamic, for example a CPU load or an Input voltage and should come to zabbix according the update interval.

On the second tab all values min, avg, and max (light green, green, pink colors) will be graphed on the graph as one horizontal pixel will "contain" more than one value.
Try to click periodically (every 2-3 sec) on a time slider to force the graph re-rendering and at some point of time you should see as between the item update interval points the Last value in the graph legend being changed.

It's rounded to an avg value for last 2-3+ values (depends on monitor size, selected interval etc) and it MISLEADS users.

---> Why when I click a time slider the Last value is "jumping" ? <---

It indeed misleads and should be fixed.

I think that the Last value in the graph legend (and probably on the graph itself) should not be rounded to an avg on any graph zoom, rendering time etc.

Confirmed for 2.0.0-2.0.7rc1 and 2.1.0 last rev 36508