Make a set of 20x100 pixel textures numbered 001.png to 100.png where 001.png has 1px grey and 99px transparent all the way up to 100.png that is completing grey.
Then use the following:
xml:
<control type="image">
<left>700</left>
<top>5</top>
<height>20</height>
<width>100</width>
<texture colordiffuse="FF12B2E7">$VAR[Percentage]</texture>
<bordertexture border="10" infill="false"></bordertexture>
</control>
And the variable:
xml:
<variable name="Percentage">
<value condition="String.IsEqual(ListItem.Property(percentage),100)">percentage/100.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),99)">percentage/099.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),98)">percentage/098.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),97)">percentage/097.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),96)">percentage/096.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),95)">percentage/095.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),95)">percentage/095.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),94)">percentage/094.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),93)">percentage/093.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),92)">percentage/092.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),91)">percentage/091.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),90)">percentage/090.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),89)">percentage/089.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),88)">percentage/088.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),87)">percentage/087.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),86)">percentage/086.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),85)">percentage/085.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),84)">percentage/084.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),83)">percentage/083.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),82)">percentage/082.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),81)">percentage/081.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),80)">percentage/080.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),79)">percentage/079.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),78)">percentage/078.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),77)">percentage/077.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),76)">percentage/076.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),75)">percentage/075.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),74)">percentage/074.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),73)">percentage/073.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),72)">percentage/072.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),71)">percentage/071.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),70)">percentage/070.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),69)">percentage/069.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),68)">percentage/068.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),67)">percentage/067.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),66)">percentage/066.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),65)">percentage/065.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),64)">percentage/064.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),63)">percentage/063.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),62)">percentage/062.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),61)">percentage/061.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),60)">percentage/060.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),59)">percentage/059.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),58)">percentage/058.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),57)">percentage/057.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),56)">percentage/056.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),55)">percentage/055.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),54)">percentage/054.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),53)">percentage/053.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),52)">percentage/052.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),51)">percentage/051.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),50)">percentage/050.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),49)">percentage/049.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),48)">percentage/048.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),47)">percentage/047.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),46)">percentage/046.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),45)">percentage/045.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),44)">percentage/044.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),43)">percentage/043.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),42)">percentage/042.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),41)">percentage/041.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),40)">percentage/040.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),39)">percentage/039.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),38)">percentage/038.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),37)">percentage/037.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),36)">percentage/036.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),35)">percentage/035.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),34)">percentage/034.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),33)">percentage/033.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),32)">percentage/032.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),31)">percentage/031.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),30)">percentage/030.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),29)">percentage/029.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),28)">percentage/028.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),27)">percentage/027.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),26)">percentage/026.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),25)">percentage/025.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),24)">percentage/024.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),23)">percentage/023.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),22)">percentage/022.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),21)">percentage/021.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),20)">percentage/020.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),19)">percentage/019.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),18)">percentage/018.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),17)">percentage/017.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),16)">percentage/016.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),15)">percentage/015.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),14)">percentage/014.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),13)">percentage/013.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),12)">percentage/012.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),11)">percentage/011.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),10)">percentage/010.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),9)">percentage/009.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),8)">percentage/008.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),7)">percentage/007.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),6)">percentage/006.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),5)">percentage/005.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),4)">percentage/004.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),3)">percentage/003.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),2)">percentage/002.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),1)">percentage/001.png</value>
<value condition="String.IsEqual(ListItem.Property(percentage),0)">percentage/000.png</value>
</variable>