尝试修改了一些bug

[liangaili]

master...liangaili:patch-1

try to fix some bugs in ecStat.js. details below …

1. Sometimes precision will become negavite value (I don't know why), set it to -precision or 0 (I don't know why, but my code works for me) if this happens.
   有的时候precision这个值会变成负数，导致出错，根据其他issue里的建议，我修改了部分precision的值，有的时候设成了-precision，有时设成了0,总之现在我这里不再出这个报错了。

2. Doing rangeArray[i - 1] + step is not right, rangeArray is an array of floats while step is a string, which makes 20 + '0.1' become '200.1', change it to (rangeArray[i - 1] + parseFloat(step));
   在计算bins的x0和x1的时候，计算到最后一个元素需要计算rangeArray[i - 1] + step，这时前项是float而后项是string，导致计算结果错误。

3. +((bin.x0 + bin.x1) / 2).toFixed(precision) will produce depulicated value, try this in your console: ((1.4+1.5)/2).toFixed(1) and ((1.3+1.4)/2).toFixed(1) will both produce 1.4
   计算bins的最后，toFixed导致了部分相邻项目会重叠。

4. var ret = +((stop >= start ? step1 : -step1).toFixed(precision)); sometimes ret will be 0 (I don't know and why), but when this happens, just return 0.1
   有时返回的step会变成0,不知道为什么，但是强行设置成了0.1可以避免出错。