Currently, this could be achieved by exporting the document to PDF and then by using RadPdfViewer's WPF control ThumbnailFactory class. Sample code may be seen at this forum post: http://www.telerik.com/forums/pdf-thumbnail-returns-transparent-images#jO33X-E8Cki_qLh_KsToWg

Add an option to save or load a custom created DocumentTheme.

Currently, this could be achieved this way:

Currently, adding hyperlinks to images is not supported.

Implement support for cell references to columns and rows, e.g. $1:$2, A:D, etc.

Currently, CellSelection.SetValue throws ParseException, and CellSelection.SetValueAsFormula throws InvalidOperationException. 

One should be able to set the axis titles in the code. The attached images show an example of the exported document opened in Excel. 

Add support for MID function. MID returns a specific number of characters from a text string, starting at the position you specify, based on the number of characters you specify.

MID(text, start_num, num_chars)

A list of the supported functions is available at http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/functions

This function can be implemented as a custom function. Check the following resources for more details on how to achieve that:

- http://docs.telerik.com/devtools/document-processing/libraries/radspreadprocessing/features/formulas/custom-functions
- https://github.com/telerik/xaml-sdk/tree/master/Spreadsheet/WPF/CustomFunctions

Add support for the Data Labels in the chart (see attached).

Expose an easy way to create .xlsx documents from DataTable objects.

When a csv is exported the entire range which has values is exported. When export is done in Excel, the cells of this range which are empty are marked by an additional field, separated by a comma. This comma is present in SpreadProcessing only in places where it is obligatory for example in the middle of the row. Obligatory comma in this context means that without it the file will not be read correctly by Excel or SpreadProcessing. In places where it is not obligatory, like in an empty row or in the end of the row, it is skipped. These non-obligatory commas do not influence how the file will be interpreted by Excel, however, according to the most-popular csv spec, they should be present, which means that there could be parsers which are unable to parse the resulting file. (2.4 Each line should contain the same number of fields throughout the file.) 

Example:
Exported by Excel:
---------------------------------------
Product,Unit Price,Units in Stock,Discontinued
Chai,$18.00 ,39,
Chang,,17,
Chef Anton's Cajun Seasoning,$22.00 ,53,No
,,,
Chef Anton's Gumbo Mix,$21.35 ,0,No
---------------------------------------
Exported by SpreadProcessing:
---------------------------------------
Product,Unit Price,Units in Stock,Discontinued
Chai,$18.00 ,39
Chang,,17
Chef Anton's Cajun Seasoning,$22.00 ,53,No

Chef Anton's Gumbo Mix,$21.35 ,0,No
---------------------------------------
Note the extra commas in the empty row and in the end of the first two rows in the SpreadProcessing sample.

Add support for conditional formatting of cells - allow applying of rules used during visualization of cells to automatically apply formatting depending on their values.

Currently, the worksheet of the shape and the collection it belongs to should be the same. Make it possible for the ChartShape and the data source to be on different worksheets.

RadSpreadsheetProcessing does not support chart color customization apart from selecting the color of the outline (set by Series.Outline.Fill). You cannot set the color of the fill of column charts etc.

Currently, row/column groups cannot be collapsed/expanded through the public API. User should be able to specify row's/column's "collapsed" attribute indicating whether the outlining is in a collapsed state.

Allow to password-protect a workbook, so that it cannot be shown (read) without the password.

Currently, the chart lines support only solid fill for their outlines.

The API should be able to provide a way to move the X-axis away from the 0 value on the Y-axis or to the bottom of the chart when there are negative Y-axis values.

In Excel this can be achieved by right click on the axis, then from the context menu select "Format Axis".  Go to the "Labels" group and then in the "Label position" field select the desired value.

The COUNTIFS function returns the count of cells that meet one or more criteria.

Sometimes the customers need to obtain the result of a formula without adding it as a value to a specific cell.

Provide an API used to recalculate all formulas in the document similar to the Excel Interop Calculate function