Hello All,
We have AVEVA Historian commissioned and working smoothly. Customer is using trend client to view the process values graphs.
The user has a requirement for trend y axis, to view grid lines, they want to specify the steps interms of values between min value and max value.
e.g. trend min value is 20.0 and max value is 25.0 and they specify the value as 0.5 then in trend grid line should be displayed at 20.5,21.0,21.5,22.0,22.5,23.0,23.5,24.0,24.5,25.0
Is it posible?
Warm regards
Hemant 
Hi Hemant,
I don't think there will be a way to do this natively, but here is how I would do it:
Open the Trend program -> Chart tab -> Scall All Tags -> Set Scale, and set the min and max:
You would then need to set the scale yourself via the Y-axis properties screen. In the example your provided, you have the min at 20 and max at 25, with a need to increment by 0.5. So, the math would be (25-20) / 0.5 = 10 increments. 
Again, I believe this is the best approach, as, to my knowledge, the grid lines cannot be directly set to increments of 0.5 through a specific input.
If you are instead trying to modify the trend during runtime using the aaHistClientTrend Control, you can then achieve a similar effect through scripting by modifying the TopY and BottomY properties:
You would then set the scale by doing similar math, and then modifying the NumYAxisValues property:
Hope this helps!
Thanks Elliott! Totally forgot that putting the chart into single mode showed the axis values.
Having set my sample chart scaling mode to be 'Single', I have identified an error in my post. The correct formula for determining the number of values within the Y-axis properties should be (Max−Min)/Increment+1. My original equation only calculates the increments and doesn't account for the fact that the trend counts the min value as one of the increments.
In the example, the Number of values property should be set to 11(Not 10), resulting in the following being displayed: 
