Weekend gaps: do they affect the correctness of our trendlines?

Since some brokers are open 7 days a week, a trendline made on such a chart (with no weekend gap) would look different from a trendline drawn on a chart with a weekend gap, right?

I guess that a good solution would be for the weekend gaps to be extended horizontally so that they occupy a 48 hours space. Is there a way to do that in MT4?

Oanda is open 7 days a week. They have the option to show or ignore weekend data. You can even choose when you want the "weekend" to start and end.

However.....
I always ignore weekend data. If you stretch 48 hours of no activity into your chart, it will throw everything off. Trendlines wouldn't work right and Moving Averages would be practically reset. You can test it out with a demo, and in the end, the choice will be whatever you decide is most helpful to you.

i think, it is better the continuation of friday to monday openning.

off-days, no noticeable differences

The last thing you want to do is (somehow) extend a 48hr gap into your MT4 charts to represent Saturday and Sunday. There are no market makers, no hedge funds, no banks and no significant traders of any kind trading at this time, nor do they count it as a part of their charts. The spot market closes 5pm EST on Friday and reopens 5pm EST on Sunday - all banks fx desks are closed outside of those standardized hours.

Trendlines - MT4 and in other packages can be impacted if your broker does not push you chart data within these times, ie., some brokers (Alpari) open at 6pm EST on Sunday. Other brokers close 4pm EST on Friday. Some don't give you data from 5pm-5:15 daily, claiming interbank data/balancing - all these scenarios can leave you with 1 or more candles less on the H1 and/or M15 charts, and obviously on all lower timeframes. More significant is pricing variance and smoothing or dealing desk control of some MT4 broker's feeds where you do not get true liquidity provider's price spikes or shakeout levels - if you are drawing a trendline or channel to these freshly printed points, later in the chart you can be tricked into believing PA is within a channel, when in fact it has already broken out. Again, an Alpari MT4 feed will have spikes tapered back. Citibank's MT4 platform and a few others are fairly good with precision price points, but for too many MT4 houses, it's trader beware. If you need a couple of examples, let me know - I have some handy.

And any gap you see on your Sunday 5pm-Friday 5pm charts is the way pros will see it too. It may or may not get 'filled' - I realize it usually does (but most pros don't give a crap about this.) And, it can go weeks without getting 'filled' - this has occurred recerntly in EURUSD.

Thanks to all! That's exactly what I wanted to know.

If you watch two charts, one with weekend charting and one without it is clear that yes they do appear extremely different on open if using any charting tools eg EMA, Bollinger's etc and they will not match again until midweek.
The only gap to pay attention to is a gap that appears in both charts.