bar diagram or line diagram? the population density of a country over 1950 to 2010 will have to represented by a suitable statistical diagram . 
which of the diagrams will be most suitable ?
I have been given two options 'multiple horizontal bar diagram' and ' line diagram ' .  Is there any harm if i use multiple horizontal bar diagram instead of line diagram ?
 A: Time is conventionally shown along the x axis, so any bars would be vertical. It wouldn't be terrible to have horizontal bars, but there is some cost ("harm") to breaking convention in that viewers will have to adapt.
It is still an interesting question to consider vertical bars versus a connected line. One factor is the number of data values (yearly, by decade?) which you don't say, but a line chart is more flexible in general:


*

*Lines can be overlaid with other lines for comparison.

*Lines scales don't have to include a zero value as bars do (though it is still often a good idea to do so).

*Lines carry a suggested continuity, as is the case with time. That is, a line segment suggests there are values between the two end points.

*Line segment slopes encode individual change amount.


Still, bars are fine if you don't need those extra features of lines. In some sense, bars are easier to read for a general audience since you don't have to worry about where the zero is on the scale.
Horizontal bars:

Vertical bars:

Line (zeroed scale):

Line (non-zeroed scale):

Another option I often use is a smoother to show the general trend with lighter dots to show the actual values:

