code: def chartkick_deep_merge(hash_a, hash_b)
        hash_a = hash_a.dup
        hash_b.each_pair do |k, v|
          tv = hash_a[k]
          hash_a[k] = tv.is_a?(Hash) && v.is_a?(Hash) ? chartkick_deep_merge(tv, v) : v
        end
        hash_a
      end
docs: https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/hash/deep_merge.rb