Tax avoidance, is "legal", but it isn't "perfectly legal"...The graph title is incorrect and misleading. They’ve lumped together evaded taxes and avoided taxes. One is perfectly legal, the other not.
The definition used for 'tax avoidance' is:
Avoidance is exploiting the tax rules to gain a tax advantage that Parliament never intended. It often involves contrived, artificial transactions that serve little or no commercial purpose other than to produce a tax advantage. It involves operating within the letter, but not the spirit of the law.
Whenever one of these articles comes out, people often incorrectly talk about ISAs and other legal tax schemes - but this is people bending the rules, or using unintentional loopholes in the law.