The line is a median trendline...I found it useful as a guide, but I might remove it if it's being interpreted as a "your mod is balanced" line. It doesn't indicate overall balance because the trend is affected by weight, i.e., a mod with several 5 fleetpoint ships and 1 15 fleetpoint ship will have a skewed trendline.
As far as armor and speed, I standby its combat performance calculation. The Onslaught is extremely effective, more so than what it's valued at. A fleet of them will destroy an equivalently priced fleet of nearly any other capital ship. My testing has thus far concluded that armor is the single greatest combat factor in large scale fleet engagements.