Fisher MetaTrader indicator — is a quite simple histogram indicator that detects direction and strength of the trend and signals trend changes. It doesn't use any standard MT4/MT5 indicators in its code. Fisher bases its calculations on the maximum and minimum price levels from the previous periods, applying some advanced math calculations to the relations between the current price and the max/min prices. The indicator is available for both MT4 and MT5. This is a "repainting" indicator — it recalculates previous bars when the new bar has arrived.
If you want to have just one line, you can remove everything related to ExtBuffer1 and ExtBuffer2, change the color of the ExtBuffer0 buffer from Black to some other color, and remove the last cycle from start() completely.
If you compare the histogram to the lines on your screenshot, you can understand that the current histogram cannot be transformed into two such lines. Histogram is really just a way to represent one line - it just changes its color when it crosses zero level.