MQL4 Reference Chart Operations ChartSetSymbolPeriod
Changes the symbol and period of the specified chart. The function is asynchronous, i.e. it sends the command and does not wait for its execution completion. The command is added to chart message queue and executed only after all previous commands have been processed.
[in] Chart ID. 0 means the current chart.
[in] Chart symbol. NULL value means the current chart symbol (Expert Advisor is attached to)
[in] Chart period (timeframe). Can be one of the ENUM_TIMEFRAMES values. 0 means the current chart period.
Returns true if the command has been added to chart queue, otherwise false. To get error details use the GetLastError() function.
The symbol/period change leads to the re-initialization of the Expert Advisor, attached to a chart. Re-initialization is not performed on offline charts, they're only refreshed (the same as when clicking Refresh in the terminal).