Mql4 Tutorial: Mastering Chart Control with ChartSetSymbolPeriod()

To change the current chart's symbol and timeframe in MQL4, you use the ChartSetSymbolPeriod function. This function is asynchronous, meaning it sends a command to the terminal queue to update the chart properties.

Below is a complete, ready-to-use script.

MQL4 Script

//+------------------------------------------------------------------+
//|                                     ChangeChartSymbolPeriod.mq4  |
//|                                  Copyright 2023, Quantitative AI |
//|                                             https://www.mql5.com |
//+------------------------------------------------------------------+
#property copyright "Copyright 2023, Quantitative AI"
#property link      "https://www.mql5.com"
#property version   "1.00"
#property strict
#property script_show_inputs // Forces the input window to appear on launch

//--- Input parameters
input string          InpSymbol   = "GBPUSD";    // Target Symbol (e.g., EURUSD)
input ENUM_TIMEFRAMES InpPeriod   = PERIOD_H1;   // Target Timeframe

//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
  {
   // 0 denotes the current chart the script is attached to
   long chart_id = 0;

   // Reset the last error code before execution
   ResetLastError();

   // Check if the symbol is available in Market Watch
   // If not, we attempt to select it to ensure data is available
   if(!SymbolSelect(InpSymbol, true))
     {
      Print("Warning: Symbol ", InpSymbol, " might not be available in Market Watch.");
     }

   // Execute the change
   // Note: ChartSetSymbolPeriod is asynchronous. It queues the command.
   bool res = ChartSetSymbolPeriod(chart_id, InpSymbol, InpPeriod);

   if(res)
     {
      Print("Successfully sent command to change chart to ", InpSymbol, " on ", EnumToString(InpPeriod));
     }
   else
     {
      // Common errors: 4106 (Unknown symbol), 4051 (Invalid function parameter)
      Print("Failed to change chart configuration. Error Code: ", GetLastError());
     }
  }
//+------------------------------------------------------------------+

Key Functions Used

1. ChartSetSymbolPeriod:

   • Syntax: bool ChartSetSymbolPeriod(long chart_id, string symbol, ENUM_TIMEFRAMES period)
   • Purpose: Changes the symbol and timeframe of the specified chart.
   • Behavior: It is asynchronous. The function returns true if the command was successfully added to the chart's message queue, not necessarily when the chart visually updates.

2. SymbolSelect:

   • Used here to ensure the requested symbol is selected in the "Market Watch" window. If a symbol is not in Market Watch, the terminal may not have the necessary tick data loaded to display the chart immediately.

3. #property script_show_inputs:

   • This directive ensures that when you drag the script onto a chart, a dialog box appears allowing you to select the InpSymbol and InpPeriod before the code executes.

Q&A

Q: Can I use this function on a specific chart ID instead of the current one?
A: Yes. Instead of passing 0 as the chart_id, you can pass a specific long value obtained via ChartFirst(), ChartNext(), or ChartID().

Q: What happens to the Expert Advisor (EA) attached to the chart if I run this script?
A: Changing the symbol or timeframe of a chart causes the terminal to re-initialize any Expert Advisor attached to it. The EA will go through OnDeinit and then OnInit again with the new symbol/period parameters.

Q: Why does the script finish before the chart actually changes?
A: ChartSetSymbolPeriod is asynchronous. The script sends the instruction to the terminal kernel and terminates immediately. The terminal processes the visual update in its own time (usually milliseconds later).