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    <p>The key is to use <a href="http://ruby-doc.org/core-1.9.3/Enumerable.html#method-i-group_by" rel="nofollow"><code>Enumerable#group_by</code></a> to find all the rows with the same month and then add them together:</p> <pre><code>require 'date' # Simulate rows from your DB Row = Struct.new(:date,:amount) rows = DATA.read.scan(/(\S+) - (\d+)/).map do |date,amount| Row.new( Date.strptime(date,'%m/%d/%y'), amount.to_i ) end sum_by_month = rows.sort_by(&amp;:date).group_by do |row| row.date.strftime('%b/%Y') end.map do |year_and_month,sub_rows| [ year_and_month, sub_rows.map(&amp;:amount).inject(:+) ] end p sum_by_month #=&gt; [ #=&gt; ["Aug/2012", 22000], #=&gt; ["Sep/2012", 5000], #=&gt; ["Oct/2012", 76000] #=&gt; ] __END__ 08/01/12 - 10000 08/16/12 - 12000 09/13/12 - 5000 10/12/12 - 76000 </code></pre> <p>For your specific use case, you could use this like:</p> <pre class="lang-html prettyprint-override"><code>&lt;table&gt;&lt;thead&gt;&lt;tr&gt;&lt;td&gt;&lt;/td&gt; &lt;%sum_by_month.each do |month,total|%&gt; &lt;th scope="col"&gt;&lt;%=month%&gt;&lt;/th&gt; &lt;%end%&gt; &lt;/thead&gt;&lt;tbody&gt; &lt;tr&gt;&lt;th&gt;Usage:&lt;/th&gt; &lt;%sum_by_month.each do |month,total|%&gt; &lt;td&gt;&lt;%=total%&gt;&lt;/td&gt; &lt;%end%&gt;&lt;/tr&gt; &lt;/tbody&gt;&lt;/table&gt; </code></pre> <hr> <p>Alternatively, here's a more direct approach (that I do not recommend):</p> <pre><code>sum_by_month = {} last_month = nil total = nil @invoices.each do |inv| month = inv.date.strftime('%b/%Y') unless last_month==month sum_by_month[month] = total if last_month last_month = month total = 0 end total += inv.amount end # Make sure to add in the very last month sum_by_month[month] = total if last_month </code></pre>
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    1. POCombine rows of m/d/y & integer for use with charts in ruby
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    1. POCombine rows of m/d/y & integer for use with charts in ruby
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    1. COhaha you must type way faster than I.
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    2. COI like this trick `sub_rows.map(&:amount).inject(:+)`!
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    3. COthanks for the code, ill dig into it tomorrow. i ended up going with HighCharts instead of Visualize, this code and the lazy highcharts gem should make this a cinch.
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