MQL4 indicator not drawing lines

Question

This is the first time I've made an indicator in MQL4. What I want it to do is draw lines like ZigZag. When I ran my indicator on the chart, I got these lines blue plotted instead:

Here is my code in the OnCalculate function:

   int Counted_bars=IndicatorCounted(); // Number of counted bars
   int i=Bars-Counted_bars-1; 
      
   // Setup
   if(rates_total<=DATA_LIMIT)
      return(0);
      
   // Loop for uncounted bars & for the 0th bar
   while(i >= 0)                   
   {
      int shift1 = i+1;
      int shift2 = i+2;
      
      double ao1 = iAO(_Symbol, 0, shift1);
      double ao2 = iAO(_Symbol, 0, shift2);
   
      // mark the start of a new range
      if(ao1 > 0 && ao2 < 0)
      {
         positiveRangeBar = BarIndex(shift1);
         highestHighVal = high[shift1];
      }
      
      if(ao1 < 0 && ao2 > 0)
      {
         negativeRangeBar = BarIndex(shift1);
         lowestLowVal = low[shift1];
      }
      
      
      // update the highest high/lowest low
      if(ao1 > 0)
      {
         double _high = high[shift1];
         if(_high > highestHighVal)
         {
            highestHighVal = _high;
            currHigh[shift1] = highestHighVal;
         }
      }
      
      if(ao1 < 0)
      {
         double _low = low[shift1];
         if(_low < lowestLowVal)
         {
            lowestLowVal = _low;
            currLow[shift1] = lowestLowVal;
         }
      }  
       
      i--;  
   }

I have no idea why this is happening.

Answer 1

From a visual review, it looks like the indicator may be using the line style, DRAW_LINE [MQL4 Reference]. This is a common line style.

Alternately, there is the DRAW_SECTION line style, which would have the MetaTrader terminal fill in any gap between non-empty* points in the indicator data. A line would be displayed in the visual model of the chart, connecting the nearest points of indicator data.

For the coding part: When using the DRAW_LINE style and given a line displaying a series of values from an indicator's data buffer in MetaTrader, if the line is not appearing at a location on the chart then the indicator's data buffer might have the value EMPTY_VALUE at that location. It may have been set to EMPTY_VALUE or simply not initialized with a "non-empty" value.

Towards understanding the logic of the indicator's data function, it looks like it might not be producing a value when _low >= lowestLowVal or _high <= highestHighVal? To have the indicator display something in these gaps, perhaps the DRAW_SECTION line style could be useful?

When developing indicators in MetaTrader, the DRAW_SECTION or DRAW_LINE style could be used with SetIndexStyle() (ref) during OnInit()

Example Code

Of course the MQL4 Reference offers examples throughout, and there are the example indicator files usually provided with the MetaTrader terminal.

If it may be of any interest as an additional point of reference towards implementing a zig zag indicator in MQL4, albeit in a programming language other than MQL4, there's a zig zag implementation in Python developed by jbn. There's a [Demo] available online.

Towards more complex applications in detecting a reversal in a market rate trend, John F. Ehlers writes about autocorrelation (Heat Map Graph) (Very Short Description, with book references).

Perhaps jbn's Python example might be more succinct? This approach does not rely on techniques in spectral analysis, digital signal processing, or applications of trigonometric functions, per se. John F. Ehlers' approach has a lot of depth though. Perhaps it's been discussed in some academic dissertations.

Footnotes

* With DRAW_SECTION, a line would be drawn between non-empty points, i.e between points in the data array that have some value other than the value EMPTY_VALUE.

The value EMPTY_VALUE has a special meaning for indicator display in MetaTrader.

In MQL4 or MQL5, an array must have some sort of value at each index. For a chart index where no data would be displayed, the indicator buffer at the index can be given the value, EMPTY_VALUE

e.g for an indicator using a double[] array named buffer_data to display data values, buffer_data[99]=EMPTY_VALUE would clear any data displayed at index 99 in that indicator.

The exact position of this data index on the chart would depend on whether the buffer_data array has been applied with ArraySetAsSeries(buffer_data, true) or alternately false.

For illustration, given the following:

ArraySetAsSeries(buffer_data, true); 
buffer_data[0] = EMPTY_VALUE;

... then the newest chart point in the indicator would have an empty value, and would typically not be displayed.

As far as I know, arrays are usually initialized in MQL4 as filled with EMPTY_VALUE. For indicators, these values would not be displayed.

Thus, if a gap appears in the indicator display, there might be an EMPTY_VALUE there, if not a value too large for MetaTrader to display the value.

The second case can happen if an EMPTY_VALUE gets used in a calculation somehow. For calculations gone towards infinity, this might be accompanied with a label such as nan (not a number) in the indicator window.

Under the proverbial hood, a double typed EMPTY_VALUE is a number, albeit a very large number that might not normally be produced from an indicator calculation. For purposes of indicator display in MetaTrader, it's interpreted as "No value" and thus may appear as a gap in the indicator's chart display.