How to use a custom format for chart series values

You can use a custom format pattern to apply to this series value, similar to the  ICU pattern set. Use something like #,### to get 1,234 as output, or $# to add dollar sign before digits. Pay attention that if you use % percentage format then your values will be multiplied by 100.

Is it possible to show dots instead of comma (e.g. 14.500,00)?

The charts use ICU patterns. And, it is not possible to reverse the decimal and thousand separators, unfortunately. Here's what it says, "Illegal patterns, such as "#.#.#" or "#.###,###", will cause DecimalFormat to set a failing UErrorCode.". Source: http://icu-project.org/apiref/icu4c/classicu_1_1DecimalFormat.html.