Problem related to chart pattern in lecture-1

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Can anybody explain this type of pattern
Actually with increase in the no of children the cost of charge should increase
but in this pattern upto 3 children there is increase in the charge and after that there is decrease in the charges

I’m assuming you’re plotting average charge.

How many people have 5 children? Probably not much, so your average is basing on a very limited set of population.
Besides, the charges are dependent per person, not the family. Perhaps (assumption) if you’re healthy enough to have and rise 5 children, you don’t need medical help that often.

Following on from Sebastian’s point, I would also recommend a boxplot (or violinplot) to see the actual distribution of charges as a “function” of number of children. It may even be worth exploring any hidden variables that correlate with number of children which could be the true cause. And just guessing here, it could be that having more children may mean your parents have more people to look after them should they need support, which in turn could minimise negative health-related issues as they grow older.