rotating horizontal axis text in multilevel

hi
I have created a chart and the horizontal axis looks like

|          |    item2_1   |  item2_2  |         |
|   item1  |            item2       an|other_ite|m

I want to rotate the text of "item1" and "another_item" however when I select horizontal axis options, only item2_1 and item2_2 can be rotated.

I have attached a picture of my chart. Is there any way to arrange the texts?