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]).

该 issue 没有指明文件、入口点或测试。首先定位 xtick 和自动 tick 生成的处理逻辑，然后明确在自动选择的范围内，纯整数 tick 应如何表现。完成的标准是：自动缩放仍保持启用，同时生成的 tick 值为整数，而不依赖数字格式化来合并非整数 tick。