Matrix-type grid - HELP?!?!

Hi all, I have a billing application in which I want to create a
matrix-type view of period billings by accounting number and by
period. It's almost working, but not quite, and I'm not Rails-savvy
enough to take it further. Can anyone point me in the right
direction? Thanks in advance! -Katie

The app is on github so you can see the models, joins, etc:
https://github.com/sdstoic/billing/tree/master/app

CONTROLLER (period_billings):

  def period_billings
    @sla = Sla.find(params[:sla_id])
  @current_period = PeriodCounter.first.period.pe_number
    @period_billings = @sla.period_billings.sort_by { |pb|
[pb.period.pe_number, pb.general_ledger.general_ledger_number] }
    @billings = Array.new(@period_billings.collect { |pb|
pb.general_ledger.general_ledger_number }).uniq.sort_by { |gl| gl }
    @periods = Period.where("fiscal_year = ?", @sla.fiscal_year)
    @general_ledgers = GeneralLedger.sla_related
    @total_period_billings =
@sla.period_billings.group_by(&:general_ledger_id)
  #@sum_current_amt =
@sla.period_billings.sum(current_amt).group_by(&:general_ledger_id)
    respond_to do |format|
      format.html # index.html.erb
      format.xml { render :xml => @period_billings }
    end
  end

VIEW (_period_billings.html.erb):

<% @cumulative_total = 0 %>

<div id="gl_crosstab">
  <table>
    <tr>
      <th>Period</th>
      <% @general_ledgers.each do |g| %>
        <th><%= g.general_ledger_number %></th>
      <% end %>
      <th>Total</th>
      <th>% Expended</th>
    </tr>
    <% @period_billings.group_by(&:general_ledger_id).each do |pb| %>
      <tr>
        <td><%= pb[1].first.period.pe_number %></td>
        <% pb[1].each do |p| %>
      <td>
      <% @sum_current_amt = pb[1].inject(0){|sum,p| sum +
p.current_amt.to_i} %>
          <%= number_to_currency(@sum_current_amt) %>
      </td>
        <% end %>
        <td>
          <% @cumulative_total = @cumulative_total + pb[1].inject(0){|
sum,billing| sum+billing.current_amt.to_i} %>
          <%= number_to_currency( @cumulative_total ) %> </td>
        <td><%= number_to_currency((@cumulative_total/
@sla.project_total)*100) %> % </td>
      </tr>
    <% end %>

  </table>
</div>