It sounds like you have the right idea. The decision whether or not to switch to Nemeth for a graphic is the same, really, as switching decisions within the narrative. A tactile graphic is not in any "code" per se -- it is the braille symbols associated with it, such as labels or keyed items, that obey the rules of UEB or Nemeth, as appropriate.
There are spacing issues in a graphic, however. If the labels can all be done in UEB, do so. If, however, a label requires a switch to Nemeth, it is ideal to place the switch indicators out of the way of the graphic -- that is, open Nemeth, blank line, graphic (with Nemeth labels), blank line, terminate Nemeth (if needed).
This will encompass the entire page only if the drawing itself takes the entire page. If the page has text before the graphic, open Nemeth after the text. The opening Nemeth Code indicator can go on the same line as the narrative, but if it doesn't fit, put it in cell 1 on the next line. The blank line the precedes the graphic will be on the following line.
An exception is made regarding Cartesian graphs, which may be transcribed without the use of code switch indicators as long as the omission is clearly stated on the Transcriber's Notes page.
Code switching needs to be considered individually with each graphic.