A really good map - like any really good data visualisation - tends to tell a story at a glance.
Sure, you might have to glance at a key to understand colours, or gradients, or whatever. But once you do, the best maps are often those where the weight, the import, hits you like a hammer.
So today, in the light of the Labour leadership contest, I decided to have a closer look at what went wrong for the party at the general election this year.
To do this, I simply looked at the vote share Labour achieved in each constituency in 2010, and the vote share it achieved in 2015.
Where Labour's vote share had gone up, I decided to colour the constituency red. Pale red for an increase of up to five percentage points. Mid-red for an increase of five to 10 percentage points. Bright red for an increase of more than 10 percentage points.
Where Labour's vote share had gone down, I decided to colour the constituency grey. Light grey for a loss of up to five percentage points, mid-grey for five to 10 percentage points, dark grey for more than 10 percentage points.
Here's the result (best viewed on desktop I'm afraid):
When I tweeted a static image of this map, I was struck by a correspondent who pointed out that it made Scotland look like a cancerous lung on a medical chart.
Certainly, from a Labour perspective, it shows that what happened in Scotland was not part of a general UK-wide pattern. What happened in Scotland was a uniquely Scottish trauma.
That's a problem for Labour. Normally, after taking a battering, you'd reflect on previous electoral history and assume you could find ways of righting the ship. But look at the map. The rest of the UK shows what a poor performance is supposed to look like: up in some places, down in others.
That sort of pattern gives you something to go on: clues about how to finesse your message.
Scotland, by contrast, was an utter wipeout. No one responded to anything Labour had to say. So what then?
Okay, so it's just one map. Just one way of looking at the figures. It certainly doesn't prove there is no way back for Labour in Scotland.
But it does suggest what happened was unique and decisive on a scale that political parties find hard to understand. Partly because they have never experienced it before. And partly, I daresay, because they'd rather live in denial.