At first glance these graphs may not appear to be a whole lot of use, but they show, for instance, how the camera's fullest mechanical potential is realized only at ISO 100 - all shutter speeds are available and the entire specified EV range (2 to 17) is utilized. They also show, and this is the most important use for these graphs, why not to use film speeds above a certain ISO outdoors in sunshine. At the South Wales coast in summer, even at ISO 200 the over-exposure warning LED may blink. On the graph (below) for ISO 200, note how the top end of the camera's operating range drops one EV to EV 16.
The GR1's fastest shutter speed is a 500th of a second, but look at the graphs to see where this point is reached. It's only at f/16 - closing the aperture to f/22 won't help once the meter's EV limit is reached. Above ISO 100, where the EV's upper limit is 17, that limit drops a notch with each step increase in ISO.
If testing the meter with ISO 200 film (for example), and the over-exposure LED blinks at f/16 (with "500" in the finder blinking), switching to f/22 does not give a steady "500" - instead, the speed readout jumps from 250 straight to an over-exposure warning and a blinking 500. In other words, above ISO 100, you cannot get f/22 at a 500th, as you might expect.
This is a physical limitation of this ultra-miniaturized system, and as long as you're aware of it, it's not a serious problem. For bright conditions, simply use ISO 100 or slower film (with slightly faster film, a little over-exposure is the same as automatic exposure compensation for brightly backlit or light colored scenes).
You can also see how many stops below a 1/30th the background illumination is for decisions about hand holding, or whether to bother with slow synchro flash. It never gets above a certain EV here (South Wales) in winter, hence it's safe to use films of up to, say, ISO 800. But summer light in a desert is very much brighter. In winter background lighting indoors here is often 5 or 6 stops below the GR1's threshold 30th using my normal film, but this shrinks to a stop or less at ISO 3200.
It's also possible to use these graphs by working backward with an SLR's built-in meter (meter an 18% grey card, a Zone V, or any handy mid-tone equivalent). The idea isn't to carry a big hand-held meter with you all the time, but rather to "get one's bearings," as it were, by doing a few readings in typical conditions.
When choosing film for outdoor use, bear in mind the old "F/16 Rule," whereby the camera's shutter speed setting is the inverse of the ISO film speed at F/16 for bright sun. For example: in the case of a GR10, which has a top shutter speed of 1/400 of a second, the rule-of-thumb says that any film over ISO 400 will be overexposed in bright sunlight. Brighter-than-normal scenes, like beaches or snow fields in bright sun, can easily add one or two EVs to the equation.
Clicking on any chart returns you here to the film speed links.
http://www.gr1.artsociety.net/
