How to convert all daily data of a year to monthly data

First let me tell you what I want, I want to make a chart of a growth of a users mutual funds portfolio.

Now I have a user who invests in a portfolio last year.

Now I have a task which stores all the daily growth values of a users
portfolio in user portfolio growth table.

Now as of current date I have all daily values of a portfolio growth in my user portfolio growth table.

What I want is to show monthly growth of a users portfolio on a graph.

So, my question here is how do I convert all the daily values to monthly values from date of his investment? So, that I can show it on a graph.

UserPortfolio Schema

# == Schema Information

#

# Table name: user_portfolios

#

#  id              :integer          not null, primary key

#  user_id         :integer          not null

#  portfolio_name  :string

#  amount          :integer          not null

#  investment_plan :string           not null

#  duration        :integer          default(1), not null

#  risk_level      :string           not null

#  created_at      :datetime         not null

#  updated_at      :datetime         not null

# == Schema Information

#

# Table name: user_portfolio_growths

#

#  id                :integer          not null, primary key

#  user_portfolio_id :integer

#  appreciated_value :float

#  created_at        :datetime         not null

#  updated_at        :datetime         not null

#