The phrase in Ichikawa's thought bubble
右肩下がり (see also
Japanese Wiktionary) literally means "right shoulder slumps/is lowered", which is a Japanese idiom to describe a decline, a graph falling on the right.
The hardest part of this translation was the texture and color of chalk on the board – it is three layers in GIMP with different filters. It looks close enough, but not exact.
Original tweet: