It's called spinning reserve, and it's the reason electricity is often so much cheaper at night. Large thermal power plants can take days to shut down and restart, so they need to keep them running anyway.
The problem can be mitigated through various means; and as problems go this is a pretty good one to have.
Storage is the obvious solution. It doesn't even need to be high quality storage, but to reduce over-generation you just need a place to dump the excess energy. You could just dump this energy as heat but optimally you'd want to recover some of it. You don't even need enough storage to carry you through the night, just absorb the over generation and shave peak. Thermal storage would work fine for that, would be relatively inexpensive and could work with existing thermal power plants. Encouraging domestic battery storage, even a few kWh worth, would also help. Almost any existing hydro could be retrofitted for axillary pumped storage.
Less obvious is to tinker with the solar panels themselves, tuning the orientation so you are optimized for late afternoon capture rather than maximum kWh/day generation. That makes the "dip" in the graph shallower and lowers the slope of the ramp.
Retire old plants that are too inflexible to meet variable demand efficiently. In other words, ditch coal.
Add usage penalties (aka "demand charges") during the ramp-up period. There are already demand charges for peak power, but spreading the demand charge out would incentivize energy efficiency and time of use habits.
Basically, there is nothing here that can't be managed with existing technology, but commercial power producers are scared shitless they'll be out of a whole lot of money. Solar is a direct threat to baseline generation (coal and nuclear) as it pushes the usefulness of peak shaving generation (gas turbine) farther into the night hours and makes baseline generation all but obsolete.
=Smidge=