The chart itself is self-explanatory: offshore wind has been catching its onshore counterpart in terms of generated clean energy. The figures above, to the end of 2018, are government ones (from here).
However at the time of writing, offshore wind capacity is already 1GW higher than at the end of 2018, has over 4GW more under construction and around 20GW in various stages along the development pipeline. From just over 8GW in 2018, the UK’s target is for at least 30GW by 2030.
By contrast, onshore wind growth in the UK risks being stalled by a current lack of policy support: strangely it cannot enter into government auctions, despite being – almost certainly – the UK’s cheapest method of power generation. Not only is new capacity uncertain, there is plenty of existing capacity coming due for retirement (or, preferably, repowering).
So there’s little doubt that offshore will soon overtake onshore; it’s just a matter of when.