Year Span Histo(ry)grams

One of the obvious limits of the first histogram is that series - or more specifically series contents, here - have an end date as well a start date; and the date span of a series is far more informative than the start date alone. So here's a first attempt at introducing date span into the visualisation. It's really a minimal tweak of the previous sketch; instead of drawing a vertical line with the histogram count (number of series commencing at a given date), I draw a stack of translucent horizontal lines from start to end year. I've also increased the scale here, so that each series line is a single pixel high; and the grid lines are now at 10 rather than 25 year intervals. Click for the full res image.

This adds a lot of visual detail, but it also obscures quite a lot. The drawing order is essentially arbitrary (it's the order of series records in the dataset as provided) and there's no collision checking, so all the lines are just overlaying each other. We can get a vague sense of the range of date spans from the top of the "spike" years, where a single stack of series lines is more clearly visible; and we can see that although the series start dates drop off sharply after 1960 (as shown in the first histogram), many series have end dates in the last 20 years.

In another quick tweak I added colour to the graph, in an attempt to pull out some of what's hidden here. By simply mapping the duration of a series (in years) to the line's hue, we can see more about the overall distribution of durations. It seems, for example, that there are a small subset of series that commence around 1900 or earlier, with very long durations. It also seems that most of the series around WWII had quite short date spans - plausible enough. So we can see a bit more here but the overdrawing problem is still significant. My next step will be to address this, perhaps by managing the drawing / stacking order to reduce overdrawing; or adding some interaction that will allow date-based highlighting of series stacks. Also in my plans is a way to stack series without any overlaps at all; a kind of packing problem. Plenty to do...


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