Bubble Chart-Issue with Y Axis

I have 2 issues, one with my Y Axis and one with the display of the bubbles.

My Y-Axis's units of Measure are percentages. My data ranges from 0% - 30%. Excel will not allow me to set the max Y axis value at .3, the minimum at .0, and the major grid-line at .15. Even when I let it auto set the max and min, it won't let me set the major gridline at .15. I figured out a work-around. I changed my data to regular number formats rather than percentages which allowed me to set the min, max, and major interval at the values that I wanted. But I would still rather use percentage so that the axis labels show %.

How do I get the bubbles to overlap the edges of the chart. The bubbles cut off at the minimum and maximum Y axis value. I have several data series with Y axis value of 0 & 30 which is my min and max. I don't want to expand the Y-axis min and max to fully display the bubbles, I would like the bubbles to overlap the plot area. I know it can be done, i've seen pictures on the internet.

Any help would be greatly appreciated. Thanks