The Join node will only output results when it receives 3 inputs, so that shouldn't be the problem.
And I was able to display the debug on the output of the Select Query function.
But nothing displayed at the MySQL debug node until I disconnect it from the Function and Dashboard Chart nodes, it works/debugs just fine.
Seems like it is the function node (which reformats array into an appropriate array for the dashboard chart) right after that caused the crash. As @dceejay pointed out, I think there is too big of a query feeding into the Function node, which caused it to crash.
I went for the approach below but I think, the output from MySQL node is still consider as one msg (a big array), therefore it still doesnt work.
I will try to work on reducing the amount of data inputting into the function/chart node, perhaps with a smooth node or rbe node. Do recommend me any other methods, I will try them out!
Thank yall so much for your help! Have a good day ahead.