# How do I fix the ngspice warning “Can't open viewport for graphics.”?

I'm trying to see some waveform plots for SPICE circuits with ngspice. As far as I can tell, the plot command is supposed to raise a window with a nice chart in it. But I keep getting this error message:

ERROR: (internal)  This operation is not defined for display type printf.
Can't open viewport for graphics.


I'm on linux mint 17.3 (ubuntu 10.04 lucid?).

I've tried:

But I'm still seeing this error message. Any ideas?

PEBACK - I was running my script in batch mode, thinking that stuff in .control would run automatically, pop up a waveform window and exit.
Instead, just ngspice circuit.cir.