The New York Times has an eye-catching graphic illustrating the Amtrak crash last year near Philadelphia. The article is here.

The various images associated with this article vary in the amount of contextual details offered to readers.

This graphic provides an overview of the situation:

Initially, I had a fair amount of trouble deciphering this chart. I was searching hard to find the contrast between the orange (labeled RECENT TRAINS) and the red (labeled TRAIN # 188). The orange color forms a wavy area akin to a river on a map. The red line segments suggest bridges that span the river bank. The visual cues kept telling me train #188 is a typical train but that conclusion was obviously wrong.

The confusion went away after I read the next graphic:

This zoomed-in view offered some helpful annotation. The data came from three days of trains prior to the accident. Surprisingly, the orange band does not visualize a range of speeds. The width of the orange band fluctuates with the median speed over those three days. And then, the red line segments represent the speed of train #188 as it passed through specific points on the itinerary.

The key visual element to look for is the red lines exceeding the width of the orange band as train #188 rounds Frankford Junction.

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In the second graphic, the speeding is more visible. But it can be made even more prominent. For example, instead of line segments, use the same curvy element to portray the speed of train #188. Then through line width or color, emphasize train #188 and push the average train to the background.

Notice that there is an additional line snaking through the middle of the orange band. The data have been centered around this line. This type of centering is problematic: the excess speed relative to the median train has been split into halves. The reader must mentally reassemble the halves. The impact of the speeding has therefore been artificially muted.

In this next version, I keep that midpoint line and use it to indicate the median speed of the trains. Then, I show how train #188 diverged from the median speed as it neared the Junction.

This version brings out one other confusing element of the original. This line that traces the median speed is also tracing the path of the train (geographically). Actually, the line does not encode speed--it just encodes the reference level of speed. The graphic above creates an impression that train #188 "ran off track" if the reader interprets the green line as a railroad track on a map. But it is off in speed, not in physical location.

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