[IMP] account: Improve graph proportions in accounting dashboard

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Purpose
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The graphs on the accounting dashboard give the impression that we had $0 in the bank because the graph is at the bottom of the kanban kard.

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Specification
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The bottom should be zero, the top should be the maximum of the visible period, and the rest of the graph should be proportionate.

addons/account/static/src/js/account_journal_dashboard_widget.js

@@ -22,6 +22,7 @@ var JournalDashboardGraph = kanban_widgets.AbstractField.extend({
                     self.$svg.addClass('o_graph_linechart');
 
                     self.chart = nv.models.lineChart();
+                    self.chart.forceY([0]);
                     self.chart.options({
                         x: function(d, u) { return u },
                         margin: {'left': 0, 'right': 0, 'top': 0, 'bottom': 0},