That chart means nothing.
It says we overpaid for Dembele, and that PSG overpaid for Mbappe, but if we reach CL finals thanks to Dembele, or win La Liga because he scored 30+ goals, or both, no one will dare saying we paid too much.
Same for Mbappe.
That chart is useless as we can only say who is worth this or that at the end of this campaign.
		
		
	 
Of course it has meaning. 
Do you really think Dembele would have cost you £130 million if PSG hadn't been putting figures of 160+ around for Mbappe? Saying the market was normal this season is just wrong, it wasn't, it was over inflated and everybody knows it.
Look at all the transfer records, they go up in increments throughout history, PSG just came in and paid double the last record. DOUBLE.
Dembele is flat out not worth what you paid, nor is Mbappe worth 180m. PSG paid that amount & for Neymar because effectively money is worthless to them since they have so much, clubs who actually are trying to balance there books (Madrid, Man United, Barcelona) won't spent 350m on two 
***UNPROVEN*** players because it makes no financial sense in the present time. 
If you really think any board of a football club would take a gamble on 1st in the league over a guaranteed 2nd in the league +200m, I can't help you.