Use a 2B pencil. Which is what you should use on a chart anyway. 2B's work fine on Imray and are easy to rub out - softer lead. HB's are too hard and will score thru the chart.
Everyones correct use a 2B pencil. But also use a very soft erase (a rubber, particularly a used one,can be very messy /forums/images/graemlins/blush.gif ) a hard erase can damage the chart surface.