After some further experimentation and verification, it turns out that what I reported above is not correct. Sorry to have caused some confusion (and perhaps some unwarranted purchases of .01 uF 6 kV tubular capacitors).
The problem with the first set of capacitors (the .005 uf 6 kV ASC tubulars) used above was that one of them was in fact bad, with basically an open circuit. On my capacitor checker, it read 50 pF. So.... when your picture looks trapezoidal with pretty bad bunching, the problem is that one of the two vertical plates is floating, and the other is doing all the work. That's seems to be the cause of the trapezoidal shape, and why there was generally not enough vertical deflection available.
And wait, it gets even more confusing. To redeem myself, I figured I would do some experiments comparing ceramic disk and tubular capacitors, to show everyone what they already know -- that ceramic caps cause problems. Well, the results show otherwise, at least in this particular set.
The various capacitors I tested are shown in the attached photos here. These are used in pairs, one for each deflection plate:
1. series array of two gold .001 uF 3 kV ceramic disk caps (should be equivalent to .0005 uF 6 kV)
2. series-parallel array of four gold .005 uF 3 kV ceramic disk caps (should be equivalent to .005 uF 6 kV)
3. single red .005 uF 4 kV ceramic disk cap (I'm actually a little over 4 kV on the HV, so this is pushing my luck a little for voltage rating, but it survived fine for the duration of the test)
4. ASC .0047 uF 6 kV tubular cap (what I normally use in the set)
5. big blue .015 uF 6.5 kV tubular cap for a laser power supply
Results in next post (not room for enough pics on this one).