Reading the picture. The gold ring is the R plasmid. The green cell carries it and survives the drug. The pale cell does not, yet. During transfer the gold line is the single strand of DNA being pumped across.
Each new recipient turns into a donor itself, so the number of donors can double every round of contact. Press the button to run more rounds.
Real populations never double this cleanly. Carrying a plasmid costs the cell energy, and not every pairing succeeds. The shape of the curve is still the point.