The Overlay function allows you to display a second symbol on an existing chart for comparison purposes.

The second symbol will only be represented as a line chart, but can be placed in the main chart section (section 1) or in a section by itself.

Open a chart, and put a symbol on it, say BHP.

Now click the Overlay (OVL) icon on the Chart Toolbar

Type in a different symbol – in this case RIO.  If you want to place the overlaid stock in a section by itself, click New Section, otherwise, click Section 1, and then press Enter, or click OK.

The second symbol is then added to the chart.

You should always take the value of the symbols into account when choosing which section in which to place your overlay, as there can be a disadvantage to adding the overlaid symbol into section 1 if the two symbols have quite different values.

See the chart we've created with BHP and RIO.  RIO being almost twice the value of BHP causes the chart to become somewhat squashed.  The effect would be even worse if the two symbols being compared were RIO and BLD (Boral), with RIO being some 1600% of BLD's price (see below).

You can get around this problem by changing the chart to a Common Base chart by ticking the appropriate box when adding the overlay.  Or, if you have already added the overlay, select Common Base from the Chart Type drop-down icon on the Chart tool bar.

The RIO/BLD chart is much more readable now that we've changed it to common base.


A Common Base chart compares the performance of the two symbols instead of their price.

When viewing a Common Base chart, you will need to specify a date from which to compare the two securities.

To do this click the Date Range section on the Chart toolbar, and scroll to select the date.  You can also simply type the start date into the Date Range section and click Apply (green tick).  The date you select is the point on the chart where both stocks equal 100 per cent, no matter what the price was for either of the stocks on that particular day.

If you don't enter a start date, the lines for the two symbols will meet on the current day.

The translation from monetary value to percentages is cascaded from the point of the comparison date to both ends of the entire chart.

The following image shows you where to click to choose the comparison date:

In the image below the comparison date has now changed to match the selection.


To clarify the percentage comparison:

Let's say the date is January 11, 2007, and BHP is trading at $20.  You want to compare it to RIO which is trading at $10 on a common base chart with the comparison date set at today (January 11, 2007).

That means that instead of BHP being $20 and RIO being $10 they now both equal 100%.

Then, tomorrow, (January 12, 2007) they both increase by $5, BHP would be trading at $25 and RIO at $15.

However, on the common base chart BHP would be sitting at 125% while RIO would be sitting at 150%.

Even though both stocks increase by $5, their percentage increase was differentbecause  $5 is 50% of $10, but only 25% of $20.


Removing overlays from a chart is done the same way as indicators.  See the Removing Indicators section for more details.


Related Topics:
Graph Explorer