What are the difference of platinum bar to gold bar?

You can't.

Do the goblin dimoplocy quest and u get 5 q points and a gold bar plus 200crafing xp.ORgo to g.e ( grand exchange ) and buy them 4 97 gp each that is 10 4 970 gpps: gold bars r rubbish they r a waste...

A gold bar is a rectangular piece of metal, usually an ingot form (an ingot is when the metal is melted and poured in a cast). Gold bars are priced based on the current price of gold per oz.

There really isn't a difference in them.

A double bar graph shows two sets f data in one chart. A bar graph just shows one.