OK, after much research, here is the answer:
You don't really need my attached updated MACD indicators because I have now found out that the MACD Histogram is already available in MT4 as the custom indicator called OsMA (shipped with MT4). They obviously wanted it in a separate file for some reason. Please note that the indicator that they call MACD somewhat confusingly shows the first MACD line as a histogram (rather than a line) and omits the real histogram.
You don't need the two MACD lines for Cowabunga just the MACD Histogram but if you want to insist on displaying all 3 in one place then here are some custom indicators giving them all combined:
MACDcorrect.MT4 for 2 the MACD lines plus histogram. This one is strictly accurate.
MACD1.MT4 (zipped) same as above but with elongated histogram to make it more readable.
I have also amended the cowabunga indicator written originally by ybop01. It wrongly used the iMACD function instead of the iOsMA function and thus seemed to miss some obvious cowabunga signals to trade. Filename COWABUNGA2.MT4 (zipped).
For anyone who remembers, MACD is composed of 3 things: the first should be a line representing the difference between two moving averages (periods 12 and 26 EMA for cowabunga) in MT4's standard MACD indicator this is confusingly displayed as a histogram, most people expect to see it as a line. The second is another slower line representing the 9 period SMA of the first line (shown correctly as a line in MT4's MACD). Thirdly is a histogram representing the difference of the first two lines (not shown at all in MT4's MACD indicator, you must use the OsMA indicator to see it).
I hope that clears this all up.
I would be glad of some feedback from anyone. Particularly anyone who has tried the original Cowabunga indicator and upgrades to this new version. There may still be improvements that can be made, it looks pretty accurate to me now, but it would be nice to hear your results (don't sue me if you lose!!!)
MACD 1.zip (1.02 KB)
MACDcorrect.zip (1.02 KB)
Cowabunga2.zip (2.28 KB)