What is a derailment?
A derailment occurs when you're not accomplishing your goal at the rate you've committed to. This happens when your datapoint for the day is on the wrong side of the red line at the deadline. (Because of how this works, you will also derail if you change your data such that your data is on the wrong side of the line for yesterday.)
To demonstrate, you can see that the top left portion of the graph below is faintly yellow, while the bottom right is plain white, and the two sides are divided by a red line. The yellow side represents what you should be achieving to stay on track. The goal in this example image has never derailed, because all the datapoints are on the "safe" (yellow) side of the line.
If your datapoint is on the wrong side of the line at your deadline, you will derail. NB: the safe side can be above or below the line, depending on the type of goal you're using.
When a derailment happens, the line will move to put you back on the safe side, and any respite period you've chosen will be added to the graph. In the example below, the graph derailed on February 13, a flat spot of 7 days was added as respite, and the line very slightly moved downwards to put the datapoint back on the right side of the line. A red triangle on the graph helps mark out when derailments happened, so you can see when you were off track.
See also our fancy tutorial which shows a bit more dynamically what derailing looks like on the graph.
Keywords: derailing, off track, derailment, getting stung, red-yesterday