Ask HN: Is there a tool to observe data and notify me about significant changes?

I’m looking for an online tool that could regularly fetch data from a given URL (it’s an API returning JSON) and notify me by email when it detects a significant change. The data usually fall into regular patterns depending on the day of the week or hour of the day. I want the tool to let me know when these patterns are disturbed. For example I want to be notified when today's values are more than 20% higher than a week before.