issue plotting Flux of a data set

Greetings.

I loaded a simple pha file ('heg_1.pha) and plotted it using
plot_data_counts() and was able to get the data to plot (on a log
scale). When I try to plot the flux of the data, I get an error in
isis:

isis> plot_data_flux(1);
Invalid: plot limits
Error: plot failed

I'm not sure what I'm missing as I'm just now learning to use isis in
my research.  Whats the best way to plot flux versus energy in isis.

Thanks,

-- 
End Transmission,
Freddy Martinez
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