Spread in the Market watch window is different from the spread drawn in the charts

Try other broker as well. Pointing to only one is not enough evidence.

1 Correct is the market watch window. Candlestick chart is incorrect.

2. No idea, my guess is they have a single feed for multiple account types. 

3. You can't, but you may add indicator that draws lines with the MQL Ask and Bid values on the candlestick chart. See https://www.forex-tsd.com/metatrader-4/32924-metatrader-known-bugs-75.html

Thank you for the answer.

1) If chart is incorrect, the historical data is incorrect (I guess) and the indicators (that I guess they use that wrong data) will show wrong values. The thing is that I can print true bid/ask lines, but the candles are being printed wrongly. What about the execution of an EA? Do they use the data coming from the chart for their internal calculations, or from the market watch? If they use the chart info, the EA behaviour will be wrong too!

2)I think so. My real account and demo account have different market watch values, but the same bid/ask in the charts.

3)The new bid/ask lines would be correct, but inconsistent with the bars and therefore very bizarre to follow the market, I think.

I've asked FXCM too to know their opinion.

I think now it's a problem of the brokers. It could be that they increase the spread of an ECN account by the commission.

Here is a example using FXCM demo server:

Maybe it is related to the fact that FXCM recently have reduced their spread. It is just like some specific config for graphs have not been updated accordingly.  

 

 

Click New Order and see if the price is correct or not.

This issue is caused by the Broker having a different feed for the Market Watch and the Charts.

The Market watch has the correct bid/ask/spread which will be used by the Broker for all order executions (deal entry triggers, stop loss and take profit)

You can use the following code snippet to show you any difference

   double chartBid = iClose(NULL, PERIOD_CURRENT, 0);
   double marketWatchBid = MarketInfo(NULL,  MODE_BID);
      
   if (chartBid != marketWatchBid)
   {
      Comment("Market Watch vs Chart :  ", DoubleToStr(marketWatchBid, Digits), " vs ", DoubleToStr(chartBid, Digits),  "  Points : ", DoubleToStr((marketWatchBid - chartBid) / Point, 0));
   }

To resolve this conflict in you indicators or EA you should only use the Market watch prices (Bid/Ask) for order execution.

This means you should change your code and replace "Bid" with iClose(NULL, PERIOD_CURRENT, 0) when comparing chart values.

But ALWAYS use MarketInfo(NULL,  MODE_BID) (or Bid for shorthand) when executing orders.