Every point is an electrolyzer design (efficiency) run at some utilization. The colour is the most it can cost per kW and still hit the target. Running it harder means buying firmed power, which costs more than the amortization it saves, so the budget peaks where the sun alone runs it.
Capex limit = (target − power price × kWh/kg) × hours run ÷ kWh/kg, with hours run = years × 8,760 × utilization and no financing or maintenance cost. The power curve is reconstructed from the post's figure: below the solar capacity factor the array is underused, above it batteries and load shifting make up the rest. 39.4 kWh/kg is the thermodynamic limit.