Apparently it was done with some sort of oscillograph.
So how come the peaks hide the drawings behind it?
Ok, thinking about this, if the drawing is done all at the same time, (like a signal FFT from the 60's) then the lower drawing device hits the upper drawing device (if the signal is bigger) hence making both trace the same thing.
The image dates from 1971. I am going to guess it was simply drawn on a plotter, using a hidden-line removal algorithm. The same could have been done on a vector monitor, but it seems the image has the wrong dimensions for that.
Actually you're both wrong. The image is from the late 60s and isn't one single plotted graph but actually multiple different measurements that have then been stacked to make comparison easier.
This article does actually explain this, but I can forgive you for not getting that far as it's the very last item on his blog and appears only to have been mentioned as an after thought.
Okay, so it was probably created in 1969 or 1970 and only published in 1971.
I think you've misunderstood what we are talking about - the method by which the graphic was created. Which I assume to have been a plotter fed instructions by a program that removed hidden lines from the graphs.
EIGHTY SUCCESSIVE PERIODS of the first pulsar observed,
CP1919 (Cambridge pulsar at 19 hours 19 minutes right
ascension), are stacked on top of one another using the
average period of 1.33730 seconds in this computer-generated
illustration produced at the Arecibo Radio Observatory in
Puerto Rico.
Apparently it was done with some sort of oscillograph.
So how come the peaks hide the drawings behind it?
Ok, thinking about this, if the drawing is done all at the same time, (like a signal FFT from the 60's) then the lower drawing device hits the upper drawing device (if the signal is bigger) hence making both trace the same thing.