MQL5 Indicator Patterns

Battle-tested patterns for creating custom MQL5 indicators with proper display, buffer management, and real-time updates.

Self-Evolving Skill: This skill improves through use. If instructions are wrong, parameters drifted, or a workaround was needed — fix this file immediately, don't defer. Only update for real, reproducible issues.

When to Use This Skill

Use this skill when:

• Creating custom MQL5 indicators for MetaTrader 5
• Debugging indicator display or buffer issues
• Setting up OnCalculate with proper warmup handling
• Implementing new bar detection patterns

Quick Reference

Essential Patterns

Display Scale (for small values < 1.0):

IndicatorSetDouble(INDICATOR_MINIMUM, 0.0);
IndicatorSetDouble(INDICATOR_MAXIMUM, 0.1);

Buffer Setup (visible + hidden):

SetIndexBuffer(0, BufVisible, INDICATOR_DATA);        // Visible
SetIndexBuffer(1, BufHidden, INDICATOR_CALCULATIONS); // Hidden

New Bar Detection (prevents drift):

static int last_processed_bar = -1;
bool is_new_bar = (i > last_processed_bar);

Warmup Calculation:

int StartCalcPosition = underlying_warmup + own_warmup;
PlotIndexSetInteger(0, PLOT_DRAW_BEGIN, StartCalcPosition);

---

Common Pitfalls

Blank Display: Set explicit scale (see Display Scale reference)

Rolling Window Drift: Use new bar detection with hidden buffer (see Recalculation reference)

Misaligned Plots: Calculate correct PLOT_DRAW_BEGIN (see Complete Template reference)

Forward-Indexed Arrays: Always set ArraySetAsSeries(buffer, false)

---

Key Patterns

For production MQL5 indicators:

1. Explicit scale for small values (< 1.0 range)
2. Hidden buffers for recalculation tracking
3. New bar detection prevents rolling window drift
4. Static variables maintain state efficiently
5. Proper warmup calculation prevents misalignment
6. Forward indexing for code clarity

These patterns solve the most common indicator development issues encountered in real-world MT5 development.

---

Troubleshooting

| Issue | Cause | Solution | | ------------------------- | ------------------------------ | ----------------------------------------------------- | | Blank indicator window | Scale not set for small values | Set INDICATOR_MINIMUM/MAXIMUM explicitly | | Values drifting over time | Rolling window not reset | Use new bar detection with hidden buffer | | Misaligned plot start | Wrong PLOT_DRAW_BEGIN | Calculate: underlying_warmup + own_warmup | | Reversed array indexing | Series mode enabled | Call ArraySetAsSeries(buffer, false) | | Buffer values incorrect | Wrong INDICATOR_DATA type | Use INDICATOR_CALCULATIONS for hidden buffers | | Compile error on buffer | Buffer count mismatch | Match #property indicator_buffers with SetIndexBuffer | | Indicator not updating | OnCalculate return wrong | Return rates_total to signal successful calculation | | Performance issues | Recalculating all bars | Only recalculate from prev_calculated onwards |

---

Reference Documentation

For detailed information, see:

• Display Scale - Fix blank indicator windows for small values
• Buffer Patterns - Visible and hidden buffer architecture
• Recalculation - Bar detection and rolling window state management
• Complete Template - Full working example with all patterns
• Debugging - Checklist for troubleshooting display issues

Post-Execution Reflection

After this skill completes, check before closing:

1. Did the command succeed? — If not, fix the instruction or error table that caused the failure.
2. Did parameters or output change? — If the underlying tool's interface drifted, update Usage examples and Parameters table to match.
3. Was a workaround needed? — If you had to improvise (different flags, extra steps), update this SKILL.md so the next invocation doesn't need the same workaround.

Only update if the issue is real and reproducible — not speculative.