At the moment, the easiest way to view historical StockRanks is to click the arrow on the Print button at the top of StockReports - you can access old StockReports and see the StockRanks there. At the top right of every StockReport you'll see the StockRank widget which contains the Quality Rank, Value Rank and Momentum Rank as well as the composite QVM StockRank.
This is basically a short term trend indicator. The delta arrow compares the StockRank now with the StockRank 30 days ago. If it has risen by more than 5 points, there will be a green arrow. If it has fallen by more than 5 points, there will be a red arrow. This gives a good indication of recent StockRank movement without being too noisy. You can hover over the arrow and see by how much the StockRank has changed over that time frame. This feature is discussed in more detailhere.
Eventually, we'll be publishing full StockRank histories, along the lines of the below, but this feature is still in progress.