It shows, for example, that although McClellan's force on the Peninsula was the weakest the Union ever fielded in the East, it was disproportionately large compared with total army strength. The large dip in summer 1864 is partially artificial, as Grant detached very significant forces to the Valley under Sheridan.
The decreasing priority given to the Eastern theatre is obvious. By the time Gettysburg is fought the Eastern theatre has barely a quarter of Union manpower. The large spike under Burnside is obvious, he had far greater forces available than any other Union commander in the East (including Grant). The massive drop in winter-spring 1863 isn't combat, nor the usual January disease losses (which usually removed as many men as a major battle), it is a massive wave of desertions that undermined the Union Army.
The effects of Grant's Overland Campaign are also obvious. He built up his forces to a level not seen since Hooker wrecked the army and then brought it down by a series of mismanaged battles that were devastating to both sides (such as the Somme was in 1916, and there is much fair comparison between Grant and Haig - BTW I think both did good jobs). The Union's voracious conscription system and employment of foreign "mercenaries" was able to keep Grant reinforced whilst Lee withered on the vine, crushed by the application of overwhelming force (as McClellan intended to do in Spring 1862).
This we can identify a "bottoming out" of resources in the East at around Gettysburg. Comparing the Eastern and Western Combat Theatres (not including the Gulf or Missouri depts in the West) looks like this:
The large jump is in March-April 1862 when McClellan's planned campaign to cut the Confederacy in two via the river systems is finally enacted (and as he moves onto the Virginia Peninsula). The gradual rise is the feeding in of continual reinforcements to try and push Grant forward towards Vicksburg, and as soon as it falls troops are pulled out and sent elsewhere. The West holds onto it's higher priority though until Grant pulls forces East to support his Overland Campaign.
If we're to look for a shift of focus then by these numbers it is Burnside's failure that inaugurates it.