I'm going to go ahead and suggest that the crossover potentially signals a market bottom, just because we've been in a steady downtrend for two months and this is the first time that something has changed.
The difficulty and hash rate are on different scales
in that graph. There is no "crossover".
But the scales are matched to where difficulty = estimated difficulty at X MH/s.
Nope, that's not what the other than red (difficulty) line means, the other 3 lines means "Computer power allocated" and that graph shows that many miners are leaving, nothing else.
(ultimately that may mean also that less coins will be generated, blocks will take longer to generate and therefore transactions will be slower to confirm)
Sorry, but you're wrong.
At the current difficulty rate (1888786), it would take about 13,520 GH/s to generate 7,200 bitcoins/day (which is the target). Guess where that red difficulty line lines up on the GH/s scale to the left?
As you can see on the chart, I have extended the red line to hit the scale on the left, for easily visualizing the scenario. I have also added tickmarks and counters for every 10 pixels on the graph. Each segment of 5,000 GH/s is 92 pixels tall.
The line hits at pixel #66. 66/92 = 71.74% of the way up from 10,000 GH/s to 15,000 GH/s. 71.74% of 5,000 GH/s = 3,587 GH/s. 10,000 + 3,587 = 13,587 GH/s. While that doesn't match exactly with my figure of 13,520 GH/s, one can only be so accurate with pixel math.
So, like I said before, the scales are matched up on purpose. If the total computing power starts to drop below the red line for a significant period of time, you can expect difficulty to be reduced accordingly.