A horizon graph is a chart that utilizes a diverging color scale, where either end of a data sequence is represented with different opaque colors that stratigraphically become more translucent and similar to the opposing color as the data reaches a start value, to showcase the different values within a dataset.
To create a horizon graph, one begins with a simple line graph that can go both positive and negative along the X or Y axis. After a line has been plotted, then a different color is used to represent each group of data within a given range. Then, all of the different color categories are re-plotted with only positive values on the graph at the axis.
An excellent example of a horizon graph can be found at flowingdata.com. In this example, the author uses a horizon graph to show the fluctuating price of bacon from 1990 to 2015. These three steps, below, show how the author shifted the data visualization from a line graph to a horizon graph:
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