Further to Brian Wood's comment – third message in this strand, inviting comments.
It is my understanding that there are two types of dial-type checking devices. They are the plunger type and the lever type, called respectively Dial Gauge and Dial Test Indicator (but sometimes misapplied). The plunger type gauge moves a rack against a pinion on the pointer, and plunger movement is proportional to what the dial shows over the whole range – and this is what the OP shows in his picture.
The lever type gives a slightly varied proportionality because the lever is more effective when the tip moves at right angles to the work, ie only over a small 'middle' section of a reading.
So, the DTI is a comparison, but the DG is a measuring tool (so I am not in accord with Brian Wood on this). Who is right, please?
PS I am not trying to score points over anyone, but I am hoping to make all our messages clear and unambiguous.
Regards, Tim