I am looking at the diagram number five How People and Trains Affect Each Other on http://mbtaviz.github.io/. I wonder how to read the legend Gray bars show entries to all stations they created. Having a look at the legend the grey colors are not separated but rather "flow" into each other. So 800 entries can either be the middle-grey-color or the dark-grey-color.
Or am I misreading it?
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The gray part of this graph is called a Horizon graph. I'm not seeing a definitive page for them but the Stephen Few blog post Horizon Graphs Revisited has some info and links to more details.
Basically, it's an area graph that's been sliced horizontally with each slice given a different color and laid over each other. For each position on the time axis, there will be one or two gray areas visible (lighter layers being obscured). The darkest area encodes the value since it represents the top of the original unsliced area graph.
This legend is too small and coarsely labeled to get much precise information from it. If we assume that the different shades of gray are starting at 0, 450 and 900, then the legend is showing that for values between 0 and 450, the graph will show a mix of white and the light gray with the amount of the second color increasing with the value. Same for the 450 to 900 range, except with the light and middle shades of graph.
So your example value of 800 will be near the top end of the middle region (450 to 900) and be represented by an area of mostly middle gray and a little light gray. The screenshot highlights the value 797, the nearest value I could find to 800.