This graph plots the Meta-Margin, defined as the amount of swing needed to create a perfect tie for Senate control. Democratic Vice-President Joe Biden, who presides over the Senate, casts tiebreaking votes. Therefore if Democrats and Independents win 50 or more seats, they will in principle be able to control the chamber.
In the polling snapshot, a tie for Senate control occurs when Democrats/Independents have a 50% probability of controlling 50 or more seats. The Meta-Margin is calculated by moving all state polls over by the same amount, M, and finding a value for M where the probability of Democratic/Independent control is 50%.
The Meta-Margin has useful applications. Do you think that polls are, overall, biased by some amount, b? Such a bias could occur, for example, if turnout is different from what pollsters as a community think it will be. If this bias b is greater than the Meta-Margin, then that would reverse who is actually favored to win Senate control.
A note on Greg Orman. The above calculation is for Democrats+Independents. If you do not like the assumption of assigning all independents including Orman to the Democrats, subtract 0.8% to assign Orman to the Republican caucus. Or subtract 0.4% to split the difference.