I've set myself a side project to scrape and display car hire price data, to compare suppliers and trends.
I've never done this before and my original goal was to play & learn new skills - but the challenge has become displaying this information in a useful way (and storing it in a sensible format).
To make it useful, I'd want to see where the prices for one supplier stand in relation to the others for all cars (a table, easy). But then the change from yesterday/last week would be useful, and seeing trends. And perhaps some stats on how the suppliers differ - is one cheaper than the others in a specific sector?
What are good resources for choosing sensible ways to display and contrast data? I've come across Stephen Few and Edward Tufte, but nothing that gives me an 'Aha!' moment yet.
Given that you've "come across" the two best resources in the field, I wonder if your lack of an "Aha!" moment is because you're not answering real questions with it.
Stephen Few lists several requirements for a dashboard to be a dashboard, and two of them are timely information and the need to look at it constantly.
Even were this not a side project, I can't imagine "car hire price data" changes minute to minute, nor that you would need to be looking at this data many times an hour and make split-second decisions based on it.
That means you're not designing a dashboard: you're designing a report. And since you don't have a real use case, you're imagining things that might be useful for an unknown user with unknown needs, and that's not how reports or dashboards work. You design a chart to answer a question, not come up with questions to ask based on a chart (we're not talking about ad-hoc visualization for exploration, here).
There isn't going to be one good way to represent all the data at once. Instead, take this to be a report, and produce a chart, graph, trend, etc. for each interesting combination. Start simple, with individual numbers, and work up to complicated contrasts. It's a side project; the only user is yourself. Let that be enough and just learn about data visualization generally.