I disagree. As you said, by 1880 the world was recovering from Little Ice Age. That is, between 1880 and mid-20th century the world showed a natural tendency

I disagree. As you said, by 1880 the world was recovering from Little Ice Age. That is, between 1880 and mid-20th century the world showed a natural tendency to become progressively warmer. This natural warming reduces the contrast with what followed, and makes the graph look more like a smooth transition. As others commented, if you look farther back, natural temperature variations smooth out, and the "hockey stick" shape becomes unmistakable.