enable setting `xtick` to Integers Only

It would be great both to have auto mode for the xticks yet being able to set them to be integer values only.

One could set them manually but them the auto scaling is gone.
Yet in some cases one would like to have only integers.
Yet setting the format to d might yield the same value number of time (1, 1.25, 1.5, 1.75, 2 will be mapped to 1, 1, 1, 2, 2 while one would like to have only 1 and 2 [Or others yet with auto range on]).

該 issue 沒有指明檔案、進入點或測試。首先定位 xtick 和自動 tick 產生的處理邏輯，然後釐清在自動選取的範圍內，純整數 tick 應如何表現。完成的標準是：自動縮放仍維持啟用，同時產生的 tick 值為整數，而不依賴數字格式化來合併非整數 tick。