For my simple test bench, I am trying to plot a voltage against a DC temperature sweep. I checked the examples, namely the test-nmos bench, for how to plot this kind of information. I include sweep=temp and set x values accordingly in the schematic plot but when reloading the raw file I see

raw_read(): no useful data found

sent to the console. I guess I am missing something in my spice directives which are attached.

You need to

set appendwrite

before writing the raw file for the second time, otherwise the op writing deletes and overwrites the dc sweep.

.control
  dc temp -15 75 5
  write tb_bandgap_opamp.raw
  set appendwrite
  op
  write tb_bandgap_opamp.raw
*  tran 0.05u 1u
*  write tb_bandgap_opamp.raw
  set nolegend
  plot all.out_se
.endc