the chart you are showing is the time it took for your synthetic to complete. if there was an error or outage you would see a red dot. There are multiple graphs and multiple levels, if you are missing some red dots, make sure you are not narrowed down to a specific location and an specific action of that synthetic monitor.
There are cases where you will not see red dots in case of errors. I have seen it in several different cases, and I give one below. This first graph is the traditional view by default:
Notice that there are few red dots. Now, in the same page, if I select "Action type"->"All actions", and then select "Performance metric"->"Total duration", I get the following graph, with much more red dots:
Please notice that besides the red dots increasing a lot, the times have changed, because "Visually Complete" and "Load duration" are completely different in this case.
It seems to me to be related with the type of errors that occur, but in any case, I would like for them to be clearly visible in the graphs.