O.K. We go round on the circle, but which circle? There are so many circles on the chart.
We have two families of circles, circles of constant resistance (blue) and circles of constant reactance (red).
Which family is more interesting to us, red or blue?
Well, it is definitely the bule.
Why?
Because, you do not want to introduce any resistance into the circuit. The resistors consume power, you know.
So all we can do is to introduce reactance while keeping the resistance constant, which means we can go around only on blue circles, right?